Rover: implement RSSI_PIN

route RSSI value from receiver to MAVLink

APMrover2/APMrover2.pde

@@ -702,6 +702,8 @@ static void medium_loop()
 		case 2:
 			medium_loopCounter++;
 
+            read_receiver_rssi();
+
 			// perform next command
 			// --------------------
 			update_commands();

APMrover2/sensors.pde

@@ -36,3 +36,12 @@ static void read_battery(void)
     }
 }
 
+
+// read the receiver RSSI as an 8 bit number for MAVLink
+// RC_CHANNELS_SCALED message
+void read_receiver_rssi(void)
+{
+    rssi_analog_source->set_pin(g.rssi_pin);
+    float ret = rssi_analog_source->read_average();
+    receiver_rssi = constrain_int16(ret, 0, 255);
+}