Added a mechanism of optional param settings for radio drivers.
Allows extra functionalities to be flexibly added to radio drivers,
with a limited overhead (3 more function pointers in the
'radio_driver' struct).
Implement the radio API extensions for nullradio.
Shows how to quickly make a Contiki radio driver aware of the optional
parameters' extensions, without much work (stub).
Refactor CC2x20 radio drivers
(allowed for fine tuning of PAN ID and addresses,
 plus removal of commented/onsolete code)
Implemented radio API extensions for CC2x20 radio drivers.
This is not a stub, but an actually useful extension, allowing upper
layers of the network stack to switch channels, set emission power
and change node addresses in a standardized way.