net: Fully disable LWIP_ASSERTs

I was doing some debugging that had me looking at the disassembly of
lpc_rx_queue() from within the debugger.  I was looking for the call to
pbuf_alloc() that we see in the following code snippet:
		p = pbuf_alloc(PBUF_RAW, (u16_t) EMAC_ETH_MAX_FLEN, PBUF_RAM);
		if (p == NULL) {
			LWIP_DEBUGF(UDP_LPC_EMAC | LWIP_DBG_TRACE,
				("lpc_rx_queue: could not allocate RX pbuf (free desc=%d)\n",
				lpc_enetif->rx_free_descs));
			return queued;
		}

		/* pbufs allocated from the RAM pool should be non-chained. */
		LWIP_ASSERT("lpc_rx_queue: pbuf is not contiguous (chained)",
			pbuf_clen(p) <= 1);

When I was looking through the disassembly for this code I noticed a
call to pbuf_clen() in the actual machine code.
=> 0x0000bab0 <+24>:	bl	0x44c0 <pbuf_clen>
   0x0000bab4 <+28>:	ldr	r3, [r4, #112]	; 0x70
   0x0000bab6 <+30>:	ldrh.w	r12, [r5, #10]
   0x0000baba <+34>:	add.w	r2, r3, #9
   0x0000babe <+38>:	add.w	r0, r12, #4294967295	; 0xffffffff

The only call to pbuf_clean made from this function is made from
within the LWIP_ASSERT.  When I looked more closely at how this macro
was defined, I saw that the mbed version of the stack had disabled the
LWIP_PLATFORM_ASSERT macro when LWIP_DEBUG was false which means that
no action will be taken if the assert is false but it still allows the
LWIP_ASSERT macro to potentially evaluate the assert expression.
Defining the LWIP_NOASSERT macro will fully disable the LWIP_ASSERT
macro.

I saw one of my TCP/IP samples shrink about 0.5K when I made this
change.
Silence GCC unused variable warning.

After making my previous commit to completely disable LWIP_ASSERT
macro invocations, I ended up with a warning in pbuf.c where an
err variable was set but only checked for success in an assert.  I
added a "(void)err;" reference to silence this warning.
net: Enable SYS_LIGHTWEIGHT_PROT

This option actually enables the use of the lwip_sys_mutex for
protecting concurrent access to such important lwIP resources as:
  select_cb_list (this is the one which orig flagged problem)
  sockets array
  mem stats (if enabled)
  heap (if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT was non-zero)
  memp pool allocs/frees
  netif->loop_last pbuf linked list
  pbuf reference counts
  ...

I first noticed this issue when I hit a crash while slamming the net
stack with a large number of TCP packets (I was actually sending 1k
data buffers from the TCPEchoServer mbed sample.)  It crashed in the
last line of this code snippet from event_callback:
  for (scb = select_cb_list; scb != NULL; scb = scb->next) {
    if (scb->sem_signalled == 0) {

It was crashing because scb had an invalid address so it generated a
bus fault.  I figured that memory was either corrupted or there was
some kind of concurrency issue.  In trying to determine which, I wanted
to walk through the select_cb_list linked list and see where it was
corrupted:
    (gdb) p scb
    $1 = (struct lwip_select_cb *) 0x85100080
    (gdb) p select_cb_list
    $2 = (struct lwip_select_cb *) 0x0

That was interesting, the head of the linked list was now NULL but it
must have had a non-NULL value when this loop started running or we
would have never gotten to the point where we hit this crash.

This was starting to look like a concurrency issue since the linked
list was modified out from underneath this thread.  Looking through the
source code for this function, I saw use of macros like
SYS_ARCH_PROTECT and SYS_ARCH_UNPROTECT which looked like they should
be providing the thead synchronization.  I disassembled the
event_callback() function in the debugger and saw no signs of the
usage of synchronizition APIs that I expected.  A search
through the code for the definition of these SYS_ARCH_UN/PROTECT
macros led me to discovering that they were actually ignored unless an
implementation defined them itself (the mbed version doesn't do so) or
the SYS_LIGHTWEIGHT_PROT macro is set to non-zero (the mbed version
didn't do this either).  Flipping the SYS_LIGHTWEIGHT_PROT macro on in
lwipopts.h fixed the crash I kept hitting, increased the size of the
code a bit, and unfortunately slows things down a bit since it now
actually serializes access to these data structures by making calls
to the RTOS sync APIs.
net: Only process 1 packet per ethernet RX interrupt

Previously the packet_rx() function would wait on the RxSem and when
signalled it would process all available inbound packets.  This used to
cause no problem but once the thread synchronization was turned
on via SYS_LIGHTWEIGHT_PROT, the semaphore actually started to overflow
its maximum token count of 65535.  This caused the mbed_die() flashing
LEDs of death.  The old code was really breaking the producer/consumer
pattern that I typically see with a semaphore since the consumer was
written to consume more than 1 produced object per semaphore wait.
Before the thread synchronization was enabled, the packet_rx() thread
could use a single time slice to process all of these packets and then
loop back around a few more times to decrement the semaphore count
while skipping the packet processing since it had all been done.
Now the packet processing code would cause the thread to give up its
time slice as it hit newly enabled critical sections.  In the end it
was possible for the code to leak 2 semaphore signals for every 1 by
which the thread was awaken.  After about 10 seconds of load, this
would cause a leak of 65535 signals.

NOTE: Two potential issues with this change:
1) The LPC_EMAC->RxConsumeIndex != LPC_EMAC->RxProduceIndex check was
   removed from packet_rx().  I believe that this is Ok since the same
   condition is later checked in lpc_low_level_input() anyway so it
   won't now try to process more packets than what exist.
2) What if ENET_IRQHandler(void) ends up not signalling the RxSem for
   every packet received?  When would that happen?  I could see it
   happening if the ethernet hardware would try to pend more than 1
   interrupt when the priority was too elevated to process the
   pending requests.  Putting the consumer loop back in packet_rx()
   and using a Signal instead of a Semaphore might be a better
   solution?
net: Reset pbuf length when re-queueing on error.

I recently pulled a NXP crash fix for their ethernet driver which will
requeue a pbuf to the ethernet driver rather than sending it to the
lwip stack if it can't allocate a new pbuf to keep the ethernet
hardware primed with available packet buffers.  While recently
reviewing this code I noticed that the full size of the pbuf wasn't
used on this re-queueing operation but the size of the last received
packet.  I now reset the pbuf size back to its originally allocated
size before doing this requeue operation.
Revert "net: Only process 1 packet per ethernet RX interrupt"

This reverts commit acb35785c90a721b0d815f91b38cc4fa9b8d157f.

It turns out that this commit actually causes problems if an ethernet
interrupt is dropped because a higher privilege task is running, such
as LocalFileSystem accesses.  If this happens, the semaphore count isn't
incremented enough times and the packet_rx() thread will fall behind and
end up running as though it had only one ethernet receive buffer.  This
causes even more lost packets.

I plan to fix this by switching the semaphore to be a signal so that
the syncronization object is more boolean.  It simply indicates if an
interrupt has arrived since the last time packet_rx() was awaken to
process inbound packets.
Don't use semaphore from ENET_IRQHandler to packet_rx

I now use a signal to communicate when a packet has been received by
the ethernet hardware and should be processed by the packet_rx thread.
Previously the change to make the lwIP stack thread safe introduced
enough delay in packet_rx that the semaphore count could lag behind
the processed packets and overflow its maximum token count.  Now the
ISR uses the signal to indicate that >= 1 packet has been received
since the last time packet_rx() was awaken.

Previously the ethernet driver used generic sys_arch* APIs exposed from
lwIP to manipulate the semaphores.  I now call CMSIS RTOS APIs
directly when using the signals.  I think this is acceptable since that
same driver source file already contains similar os* calls that talk
directly to the RTOS.
Changed leading whitespace back to tab.

The leading whitespace preceeding the fields in the lpc_enetdata
structure definition were originally a tab and I used 4 spaces when
I added RxThread.
Merge pull request #46 from adamgreen/netAssertDisableAndCrashFixes2

Robustness fixes for netstack