Rover: motor_active method similar to the plane is_flying but simpler

APMrover2/Rover.h

@@ -372,7 +372,7 @@ private:
     uint32_t rudder_arm_timer;
 
     // true if we are in an auto-throttle mode, which means
-    // we need to run the speed/height controller
+    // we need to run the speed controller
     bool auto_throttle_mode;
 
 
@@ -522,7 +522,7 @@ private:
     void change_arm_state(void);
     bool disarm_motors(void);
     bool arm_motors(AP_Arming::ArmingMethod method);
-    bool is_moving();
+    bool motor_active();
     void update_home();
 
 public:

APMrover2/control_modes.cpp

@@ -117,3 +117,14 @@ void Rover::read_trim_switch()
     }
 }
 
+bool Rover::motor_active()
+{
+    // Check if armed and throttle is not neutral
+    if (hal.util->get_soft_armed()) {
+        if (!channel_throttle->in_trim_dz()) {
+            return true;
+        }
+    }
+
+    return false;
+}

APMrover2/radio.cpp

@@ -49,8 +49,10 @@ void Rover::init_rc_out()
     // setup PWM values to send if the FMU firmware dies
     RC_Channel::setup_failsafe_trim_all();  
 
-    // setup PX4 to output the min throttle when safety off if arming
-    // is setup for min on disarm
+    // output throttle trim when safety off if arming
+    // is setup for min on disarm.  MIN is from plane where MIN is effectively no throttle.
+    // For Rover's no throttle means TRIM as rovers can go backwards i.e. MIN throttle is
+    // full speed backward.
     if (arming.arming_required() == AP_Arming::YES_MIN_PWM) {
         hal.rcout->set_safety_pwm(1UL<<(rcmap.throttle()-1),  channel_throttle->radio_trim);
     }
@@ -94,8 +96,8 @@ void Rover::rudder_arm_disarm_check()
 			// not at full right rudder
 			rudder_arm_timer = 0;
 		}
-	} else if (!is_moving()) {
-		// when armed and not moving, full left rudder starts disarming counter
+	} else if (!motor_active()) {
+		// when armed and motor not active (not moving), full left rudder starts disarming counter
 		if (channel_steer->control_in < -4000) {
 			uint32_t now = millis();