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>
lm4f: Add USB bulk device example

I wrote this example to benchmark the USB driver. See the README.md for
full details.

Why README.md instead of just README. README.md shows up better on github,
and makes it easier to browse the tree on github to search for an example.

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