Plane: allow throttle failsafe on no RC input

on PX4 we just stop getting input on loss of RC - we need to consider
this to be "throttle failsafe"

ArduPlane/ArduPlane.pde

@@ -408,6 +408,8 @@ static struct {
 
     // A timer used to track how long we have been in a "short failsafe" condition due to loss of RC signal
     uint32_t ch3_timer_ms;
+
+    uint32_t last_valid_rc_ms;
 } failsafe;
 
 

ArduPlane/radio.pde

@@ -111,16 +111,21 @@ static void rudder_arm_check()
                 Log_Arm_Disarm();
             }                
         }
-
-    }
-    else { //not at full left or right rudder
+    } else { 
+        // not at full right rudder
         rudder_arm_timer = 0;
-        return;
     }
 }
 
 static void read_radio()
 {
+    if (!hal.rcin->valid_channels()) {
+        control_failsafe(channel_throttle->radio_in);
+        return;
+    }
+
+    failsafe.last_valid_rc_ms = hal.scheduler->millis();
+
     elevon.ch1_temp = channel_roll->read();
     elevon.ch2_temp = channel_pitch->read();
     uint16_t pwm_roll, pwm_pitch;
@@ -289,6 +294,10 @@ static bool throttle_failsafe_level(void)
     if (!g.throttle_fs_enabled) {
         return false;
     }
+    if (hal.scheduler->millis() - failsafe.last_valid_rc_ms > 2000) {
+        // we haven't had a valid RC frame for 2 seconds
+        return true;
+    }
     if (channel_throttle->get_reverse()) {
         return channel_throttle->radio_in >= g.throttle_fs_value;
     }