Radio api extension

Allows extra functionalities to be flexibly added to radio drivers, with a limited overhead (3 more function pointers in the 'radio_driver' struct). These extensions can be enabled/disabled with a preprocessor option (RADIO_CONF_EXTENDED_API).

The three added functions are :
* <code>get_config_const()</code>
  get configuration constant(s) of the radio transceiver.
* <code>set_param()</code>
  set a config parametre of the radio transceiver.
* <code>get_param()</code>
  get a config parametre of the radio transceiver.

The semantics of these functions are explained in the code comments.