First start calculating the CRC in hardware when receiving a frame.
Then, other than the at86rf2xx transceivers the cc2420 don't consider the
CRC_OK flag when reporting a successful packet reception. This change
introduces a check for this bit and drops the packet else by flushing
the RX FIFO.
Instead of using differing integer types use kernel_pid_t for process
identifier. This type is introduced in a new header file to avoid
circular dependencies.
This PR converts tabs to white spaces.
The statement I used for the conversion:
```find . -name "*.[ch]" -exec zsh -c 'expand -t 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;```
Afterwards, I had a quick overview of the converted files to prevent odd indentation.
