Intermediate commit, working towards wireless flashing.

Refactor the pov_sender.c code to grab one packet at a time from USB;
this makes it easier to pass a debug packet over to a different
routine.

Also make corruption detection a bit more robust.
Introduce some #define's for the hardware pins and such used to communicate with the nRF24L01+

This does not fully allow to change pins arbitrarily (for example there is
unlock needed for some pin, and the IRQ handler also would need changing), but
it makes it somewhat nicer to add more code that references the SSI base
address and pins and such.
First working flash over wireless.

This version successfully can flash a test app on the target over the wireless.
(This part is the sender side, working as a USB-connected programmer on the PC).

There is still some glitches when connecting to the bootloader, if the
bootloader is not already running when wireless-flash.pl starts. This
needs to be fixed later. For now, to flash, reset the target (so it
enters the bootloader) and start the programmer while the bootloader
is still listening for packets.