Extended radio API with support for setting channel, pan id, addressing modes, etc

Extended radio API developed during implemention of IEEE 802.15.4 active scan for automatic network configuration. The extended radio API as shown below has been adapted based on the discussions in #519.

```
typedef int radio_value_t;
typedef unsigned radio_param_t;

struct radio_driver {
  [...]
  radio_result_t (* get_value)(radio_param_t param, radio_value_t *value);
  radio_result_t (* set_value)(radio_param_t param, radio_value_t value);

  /** Get a radio object (for example a 64 bit address) */
  radio_result_t (* get_object)(radio_param_t param, void *dest, size_t size);
  radio_result_t (* set_object)(radio_param_t param, const void *src, size_t size);
}
```
Currently full support for STM32W, limited support for CC2420/CC2520, and stubs for other platforms.