Working usb-serial tx only
Switch from polled to autoserving usb interrupts
Bidirectional serial now.

Just missing the dma to tx, and the rs485 control pins!
Add docs on lm4f implementations
Enable DMA for tx direction
Use new rcc api
Add support for setLineCoding
Fix up RX leds and names
Turn off leds/pins when tx finishes as per app note