Added a way to obtain the RSSI from the CC2538 RF core
Added a function for setting promiscous mode on the CC2538 RF core
Added header declarations for cc2538_rf_read_rssi() and cc2538_rf_set_promiscous_mode()
Added missing argument declaration
Merge pull request #458 from adamdunkels/push/cc2538-rf

CC2538 RF core additions