Work on second mbed

Work on second mbed

Add control program file:
  - Can be used to send a time initialization message to the mbed

Add install program:
  - Install programs on both mbeds and initialize the time on one

Add IRQ driven serial interface:
  - Since we don't have spare DMA controllers available for this
    homework we need to transmit serial data some other way.

  - The IRQ code allows us to queue up large messages to be transmitted
    asynchronous to main background loop. It should also help with
    receiving data at a high rate.

Add task table to main:
  - This lets us schedule tasks at fixed intervals.

Fix various bugs:
  - Timer dma code reset avoid config errors cause by periodical request
    while the DMA controller has nothing to transmit.