AP_AHRS: reject EKF for plane when we have GPS and aren't fusing

plane users would prefer to use GPS in this case

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -324,6 +324,13 @@ bool AP_AHRS_NavEKF::using_EKF(void) const
 #if APM_BUILD_TYPE(APM_BUILD_ArduPlane) || APM_BUILD_TYPE(APM_BUILD_APMrover2)
     nav_filter_status filt_state;
     EKF.getFilterStatus(filt_state);
+    if (hal.util->get_soft_armed() && !filt_state.flags.using_gps && _gps.status() >= AP_GPS::GPS_OK_FIX_3D) {
+        // if the EKF is not fusing GPS and we have a 3D lock, then
+        // plane and rover would prefer to use the GPS position from
+        // DCM. This is a safety net while some issues with the EKF
+        // get sorted out
+        return false;
+    }
     if (hal.util->get_soft_armed() && filt_state.flags.const_pos_mode) {
         return false;
     }