lm4f: Improve read/write endpoint performance

Modify lm4f_ep_read/write_packet() to take advantage of 32-bit and 16-bit
accesses to the USB FIFO, as opposed to using only 8-bit accesses. This
change increases endpoint performance in a number of configurations.

On the host side, we use usb_bulk_bench to test
https://github.com/mrnuke/usb_bulk_bench

With the following invocations:

usb_bulk_bench -d c03e:b007 -a -t 64 -q 32 -e [ep] [-I/-O]

On the device side, we use the usb_bulk_dev example:

https://github.com/mrnuke/libopencm3-examples/tree/stellaris
The example is in: examples/lm4f/stellaris-ek-lm4f120xl/usb_bulk_dev

(This example will be available in libopencm3-examples in the near future)

The endpoints configuration is the following:
* EP1 OUT - interrupt driven RX endpoint
* EP2 IN  - interrupt driven TX endpoint
* EP3 OUT - polled RX endpoint
* EP4 IN  - polled TX endpoint
* EP5 OUT - polled RX endpoint with unaligned buffer
* EP6 IN  - polled TX endpoint with unaligned buffer

We test the speed in each configuration, using different system clock
frequencies. We run the tests once without the patch applied, and once with
the patch applied. The results are given below:

Before patch (numbers in KiB/s):
freq:	80 MHz	57 MHz	40 MHz	30 MHz	20 MHz	16 MHz
EP1	562	562	562	562	562	550
EP2	936	872	812	812	687	625
EP3	1062	890	700	600	562	562
EP4	900	812	812	750	625	562
EP5	1062	890	700	600	562	562
EP6	930	812	812	750	625	562

With patch  (numbers in KiB/s):
freq:	80 MHz	57 MHz	40 MHz	30 MHz	20 MHz	16 MHz
EP1	1062	1062	1062	690	562	562
EP2	1125	936	936	936	870	812
EP3	1062	960	750	750	562	562
EP4	936	936	870	870	770	700
EP5	1062	900	700	630	562	562
EP6	930	930	870	870	740	650

Percent change in speed (*):
freq:	80 MHz	57 MHz	40 MHz	30 MHz	20 MHz	16 MHz
EP1	89.0	89.0	89.0	22.8	0.0	2.2
EP2	20.2	7.3	15.3	15.3	26.6	29.9
EP3	0.0	7.9	7.1	25.0	0.0	0.0
EP4	4.0	15.3	7.1	16.0	23.2	24.6
EP5	0.0	1.1	0.0	5.0	0.0	0.0
EP6	0.0	14.5	7.1	16.0	18.4	15.7

(*) Numbers given as percent change relative to speed before applying this
patch.

We see throughput increases across the board.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Switch GPIOs to using AHB bus ***WARNING***

The AHB bus allows faster control of GPIO pins versus the older APB bus.
The GPIO ports A through H default on the APB bus. Change the GPIOx
defines in gpio.h to use the base address of the AHB bus.

There's another reason to use the AHB bus: ports K tand highercan only be
accessed via the AHB bus.

***WARNING***
To work, GPIO acces to the AHB bus must be explictly enabled via the
GPIOHBCTL register. Without any additional change, this patch will break
any code using the older APB bus. If the GPIOHBCTL is not properly
modified, any acces to the GPIO register will cause a hard fault.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: "Pretty-ize" the look of Doxygen comments in gpio.c/.h

Put the Doxygen blocks in well-formatted C comments. This looks better
and is less distracting than putting them in unformatted comments.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add all GPIO ports to memorymap.h

The base addresses for GPIO ports J through Q were missing. Add them.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add GPIO register definitions

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Implement GPIO configuration and control

Everything needed to set up and control the GPIO pins is implemented,
EXCEPT setting up interrupts. This is the subject of a subsequent patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Implement GPIO interrupt control

Implement an API to specifiy the interrupt trigger for GPIO pins, and
control interrupts. This completes the GPIO API.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Split sam3x/sam3n support.
sam: Add convenience function for clock setup using internal 4MHz.
lm4f: Add control over UART FIFOs
[make] Use less shell variables

Use fixed variables for things that can't change, instead of variables
like SRCLIBDIR that was causing a shell call for every single use.

Use less functions in general and less shell expansions.

Use builtin functions like $(realpath blah) instead of $(shell
pwd)/blah  In particular, this was important for me trying to build on
windows with git-shell.  This should help a lot on smaller build
machines, but doesn't make a huge difference on my own.

Remove redundant clean print messages. These also cause more needless
shell expansions.