Sub: rename system_status as it won't be called from base class

ArduSub/GCS_Mavlink.cpp

@@ -62,7 +62,7 @@ uint32_t GCS_Sub::custom_mode() const
     return sub.control_mode;
 }
 
-MAV_STATE GCS_MAVLINK_Sub::system_status() const
+MAV_STATE GCS_MAVLINK_Sub::vehicle_system_status() const
 {
     // set system as critical if any failsafe have triggered
     if (sub.any_failsafe_triggered())  {

ArduSub/GCS_Mavlink.h

@@ -50,7 +50,7 @@ private:
     bool send_info(void);
 
     MAV_MODE base_mode() const override;
-    MAV_STATE system_status() const override;
+    MAV_STATE vehicle_system_status() const override;
 
     int16_t vfr_hud_throttle() const override;