move cc2420 hardware aes support to cc2420.c

ContikiMAC can preempt the standalone encrytion process and access the
SPI at the same time as the encryption process. As a result, faulty AES
results are returned.
In order to lock interactions with the radio, we need to move the
standalone encryption to cc2420.c. Alternatively, the lock could be made
public. However, moving all cc2420-related functionality to one file
was deemed the cleaner way.
move CC2420_WRITE_RAM_REV to header

this is where similar defines are located as well.
use accessor functions
conditionally busy wait until timeout occurs

this prevents indefinite waiting if the hardware doesn't report
back correctly.
add simple hardware aes test example

This example runs hardware aes several times in a row and evaluates the
output of a hardware AES operation for fixed a key and input. If the
output deviates, this may be an indication that other protothreads
interfere with this protothread.