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>