sensnif compatible peripheral code

This uses simrf to implement a https://github.com/g-oikonomou/sensniff
compatible peripheral, using the "new" payload format documented on that
page.  (Current as of revision b02516d302b034a44a682e0fb4930933ee5d7045)

MRF24J40 only captures frames with valid FCS, so we just create "TI
CC25xx FCS" format ourselves from the RSSI/LQI fields the MRF24J40
provides.

This was tested on the STM32VL Discovery Board, but it's the same code
for the STM32L disco board too.