Extended radio driver API

Extensions to the radio driver API, so as to allow radio transceivers' specific features (e.g.: channel switching, emission power setting, node address changing) to be provided a standardized way by drivers.
As examples, 3 drivers (included in core) have been upgraded to use these extensions: 'nullradio' is a stub (but shows how to quickly and easily make a driver compliant with these extensions), while 'CC2420' and 'CC2520' are actually offering new features using these extensions.