Extended RF API: CC2530 driver, minor CC2538 and example changes

* Implementation of extended API functions for the CC2530 RF
* Small improvement to the CC2538 driver
* Example change (below)

For the example, we want to be able to test 64-bit addresses, therefore `LINKADDR_CONF_SIZE` must be 8. Originally I achieved this by enabling IPv6 for the example, but ideally I'd like to be able to run the example on some older hardware (namely Sensinode). The change proposed here disables IPv6 and sets `LINKADDR_CONF_SIZE` to 8 in the conf.

Please test changes to the example with your kit before merging. Tested successfully with CC2530 and CC2538