nrfProg for Raspberry Pi

[SuperFlue]

It might be that it already works since I've not gotten around to test it.
But it would be nice if nrfProg was working with the Raspberry Pi.

As far as i can see the Amontec JTAG Key is not that easy to get a hold of anymore, but the Raspberry Pi is easy to get and supports connecting to devices over SPI.

[ataffanel]

There is a script to recover the bootloader using a raspberrypi in case the firmware cannot start it (https://github.com/bitcraze/crazyradio-firmware/tree/master/fix_bootloader).

Porting nrfprog to use the raspberrypi GPIO and SPI port would be a good idea I think. Looking at the code briefly it is already be designed to be ported, the main thing that needs to be done is to re implement the functions defined in nrfProg/nrfSpi.h.