AP_Compass: Backend: add method is_external()

libraries/AP_Compass/AP_Compass_Backend.cpp

@@ -117,3 +117,8 @@ void AP_Compass_Backend::set_external(uint8_t instance, bool external)
         _compass._state[instance].external.set(external);
     }
 }
+
+bool AP_Compass_Backend::is_external(uint8_t instance)
+{
+    return _compass._state[instance].external;
+}

libraries/AP_Compass/AP_Compass_Backend.h

@@ -72,6 +72,9 @@ protected:
     // set external state for an instance
     void set_external(uint8_t instance, bool external);
 
+    // tell if instance is an external compass
+    bool is_external(uint8_t instance);
+
     // access to frontend
     Compass &_compass;