AP_Vehicle: move control_mode_reason up to AP_Vehicle

libraries/AP_Vehicle/AP_Vehicle.h

@@ -76,9 +76,14 @@ public:
     // HAL::Callbacks implementation.
     void loop() override final;
 
+    // set_mode *must* set control_mode_reason
     bool virtual set_mode(const uint8_t new_mode, const ModeReason reason) = 0;
     uint8_t virtual get_mode() const = 0;
 
+    ModeReason get_control_mode_reason() const {
+        return control_mode_reason;
+    }
+
     /*
       common parameters for fixed wing aircraft
      */
@@ -321,6 +326,8 @@ protected:
     void publish_osd_info();
 #endif
 
+    ModeReason control_mode_reason = ModeReason::UNKNOWN;
+
 private:
 
     // delay() callback that processing MAVLink packets