Extended radio drivers API

Extensions for the radio drivers API, so as to allow specific radio transceivers' features to be provided a standardised way, using a mechanism of optional parameters. Among these (useful) specific features, we can cite: channel switching, emission power setting, node address changing.
As example, 3 drivers have been upgraded to use these extensions: 'nullradio' is a stub (however it shows how to quickly, safely and easily make an existing driver aware of this extension mechanism) while 'CC2420' and 'CC2520' are actually providing new features using our optional parameters.
Note however, that all these extensions are disabled by default (using preprocessor and a configuration variable).