add interrupt code for RSSI reporting in RF12demo

- RF12demo now reports RSSI as last value, if supported
- example: OK 3 54 175 25 0 (-41)
- power up default now starts with quiet flag set (“1q”)
- default in GitHub now has DATAFLASH set to 0
- version number bumped to 11, i.e. “[RF12demo.11]”