TODO

- fix license for printf.c (take from newlib instead of glibc)

- fix license for start.S (find a BSD version)

- openocd flashing
  use run_algorithm to hook ROM NVM functions
  see flash/stm32x.c

- add timestamp to received packets 

- add a way to set modes: tx_only, rx_only, txrx. (so you can be lazy
  and have blocks of code that don't need to handle rx packets)

- beacon sync

- CCA and ED.

- auto-acking and ack managment
  keep a set of bit fields, one bit per sequence number (DSN), so 32 bytes
   total.
  Set them all to 1 initially. When a DSN is sent, clear the bit. Set
   the bit when the ack is received.
  Do anything special with sent but unacked packets? Maybe, so you can
   retransmit them.

  Dosen't look like contiki ever uses 802.15.4 acking: grep ACKFRAME

  not much point to implement it here unless you also plan to use in
   in contiki (pretty substantial change to core Contiki).

 also you ack the data after checking it's for you. so autoack relies
 on setting up the hardware addressing.