Biggest problem was definitions in rtimer_arch.h -- we have a 16-bit
rtimer_clock_t so this was overflowing and not working. Therefore most
delays in the radio implementation didn't work.
Some implementations don't have enough pins to implement CC2520_FIFO on a
separate pin. By not defining CC2520_FIFO_IS_1 an alternative
implementation uses an spi command to retrieve the FIFO overflow
exception info.