AP_NavEKF3: allow switch to ExtNav from optflow

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -225,7 +225,7 @@ void NavEKF3_core::setAidingMode()
             bool bodyOdmFusionTimeout = ((imuSampleTime_ms - prevBodyVelFuseTime_ms) > 5000);
             // Enable switch to absolute position mode if GPS or range beacon data is available
             // If GPS or range beacons data is not available and flow fusion has timed out, then fall-back to no-aiding
-            if(readyToUseGPS() || readyToUseRangeBeacon()) {
+            if (readyToUseGPS() || readyToUseRangeBeacon() || readyToUseExtNav()) {
                 PV_AidingMode = AID_ABSOLUTE;
             } else if (flowFusionTimeout && bodyOdmFusionTimeout) {
                 PV_AidingMode = AID_NONE;