Implement extended RF API for the CC2530 RF
Don't hardcode min and max TX power values
Use the extended RF API for the cc2530 border router example
Improve the CC2538 extended RF API implementation

* Decouple 64-bit address from LINKADDR_SIZE
* Read out the correct number of bytes when getting the 64-bit address
* The RF no longer sets parameters by itself. We let the platform do this, using the extended API.