Make the CC2538 UART driver more configurable

* We can now very easily switch between UART0 and UART1 through a define
* We can also configure the UART RX and TX port/pin through defines
Remove leftover make targets

The sensinode platform does not support .upload and .serialdump
Their presence in the makefile has confused in the past confused
some users. This commit removes them

The commit also removes the $(OBJECTDIR)/%.rel: %.cS recipe which
is not used by either 8051 platform and is probably broken anyway,
since it has been unmaintained for years
Auto-enable SLIP for 8051-based platforms
Move SLIP interface define to project-conf
Disable example-defined putchar for 8051 platforms
Increase 8051 plaform configurability
No longer build rpl-of0 for Sensinodes
Change the cc2531 usb stick descriptor

The CC2531 USB stick now identifies itself as a
'Texas Instruments CC2531 USB Dongle' and uses a
TI-assigmed VID:PID. The VID:PID is now configurable
in contiki- or project-conf.h
Change the way we switch between cc2530 models

The switch is now performed through a configuration define,
which has also been renamed to better match Contiki's naming
conventions
Don't build the Sensinode UDP server as RPL root

The Sensinode UDP-IPv6 example used to build the
UDP server to act as RPL root by default. This now
changes, the server is a simple router. Support to
build the server as RPL root is still there, it's
just the default that changes
Don't redirect sdld output to dev/null

This was used in the past because sdld was
very verbose when linking banked hex files. New
sdld versions do not exhibit this level of
verbosity and therefore the redirect can be
stopped
Change to sdar from the now deprecated sdcclib
Support verbosity control for 8051-based platforms
Disable SDCC warnings 110 and 126

Those two warnings are optimisation-related
* 110 warns that an always-false if branch has been optimised out
* 126 warns about unreachable code which also gets optimised out

In disabling those warnings, we make the build less cluttered
Fix cc2530 UART1 init bug

Fixes #286
Remove the second 6LoWPAN context from Sensinode

This context was once used in an in-house testbed
and is not required upstream
Decrease Sensinode example code size
Merge pull request #329 from g-oikonomou/8051-updates

Various Updates for 8051-based Platforms
Merge pull request #328 from g-oikonomou/cc2538-uart-config

Make the CC2538 UART driver more configurable
6lowpan fragmentation bugfix: the 6lowpan code had an unfortunate
interaction with the behavior of the rdc layer. If the first packet of a
fragment transmission was lost, the remaining packets would get dropped
on reception. Moreover, the reception code contained a bug that
sometimes would cause fragments to be misidentified as fragments. Taken
together, these problems would result in a pathelogical network
breakdown if too many fragmented packets would occur simultaneously.