CC2420: Added RSSI_OFFSET as described in the manual
CC2420: Added RSSI_OFFSET as described in the manual

MohamedSeliem/contiki,bluerover/6lbr,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr,MohamedSeliem/contiki,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,arurke/contiki,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,bluerover/6lbr,MohamedSeliem/contiki,arurke/contiki,arurke/contiki