lm4f: Add basic UART echo example

Nothing fancy, just a demonstration of the blocking send and recieve.

Also update libopencm3 submodule to include the lm4f UART API.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add UART echo by interrupt example

Nothing fancy, just a demonstration of the using the UART interrupt service
routine.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Updated libopencm3.
lm4f: Update makefile for "make flash" support

When the makefile was originally written, openocd did not officially
support ICDI. Now that ICDI support is part of an official openocd release,
update the makefile to enable flashing with "make flash".