This example is for appearing as a CDC-AMC "serial comm port".

 * (TBD) loopback - Echo everything back to you (useful for testing).

 * to-uart - Sends data in and out using the hardware UART.

 * (TBD) to-pin - Sends data in and out using bit banging software UART.

 * (TBD) to-fifo - Sends data in and out using the FIFO system.
   This is the highest performing mode, getting up to 30mega*bytes* per second.

This current implementation doesn't correctly send the periodic INT transfers
with the line state information. This should be added to endpoint 1.

TBD: To be done.
