AP_NavEKF: Prevent bad GPS pre-arming casuing initial position errors

If the vehicle moves significantly or the GPS changes position significantly pre-armed, then the GPS glitch logic was being invoked when the first GPs measurements were fused. This patch resets the position to the GPS when the vehicle arms.

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -4621,6 +4621,8 @@ void NavEKF::performArmingChecks()
                 PV_AidingMode = AID_ABSOLUTE; // we have GPS data and can estimate all vehicle states
                 constPosMode = false;
                 constVelMode = false;
+                // we reset the position in case the initial GPS setting the origin was off or the vehicle has been moved a significant distance
+                ResetPosition();
             }
         }
         // Reset filter positon, height and velocity states on arming or disarming