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".
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>