lm4f: Add USB to serial (CDCACM) example

Implements a USB-to-serial adapter, compliant to the CDCACM subclass.
UART1 is used for the TX/RX lines. Although UART1 also has lines for flow
control, they are not used. Instead, the control lines are implemented in
software via GPIOA.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>