EKF: Enable origin to be maintained when starting aiding using EV only
EKF: Enable origin to be maintained when starting aiding using EV only

When starting aiding using EV only and commencing GPS aiding later, this change means that the GPS origin is set to the local position 0,0 point rather than the current vehicle position. This avoids large changes in local position when GPs aiding starts.

PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl