Copter: bypass compass healthy check when ext nav data is available for heading

ArduCopter/AP_Arming.cpp

@@ -456,12 +456,15 @@ bool AP_Arming_Copter::arm_checks(AP_Arming::Method method)
     }
 
 #ifndef ALLOW_ARM_NO_COMPASS
+    // if external source of heading is available, we can skip compass health check
+    if (!ahrs.is_ext_nav_used_for_yaw()) {
         const Compass &_compass = AP::compass();
         // check compass health
         if (!_compass.healthy()) {
             check_failed(ARMING_CHECK_NONE, true, "Compass not healthy");
             return false;
         }
+    }
 #endif
 
     control_mode_t control_mode = copter.control_mode;