Very basic wireless flashing through an nRF24L01+ radio.

Run make with RADIO_UART=1 to enable, this will check if the chip is
actually connected when starting, and normal operation over UART should
still work.  There are still a few issues to fix with this code.  It
makes the binary bigger by about 800 bytes (nearly triples the
bootloader size).
An ugly implementation of the BIGBOOT=1 flag, which till now didn't work at all.

Now at least it kinda-works.
Move write buffer to make space for .bss section.

With RADIO_UART we're using a few static variables so we do actually
need a .bss section, which was not needed until now.
Clean our new .bss on start.
Implement SUPPORT_EEPROM.

Tested to work, adds about 140 bytes to the code size (I do wonder why
that much).