ekf2: relax zero velocity update

The ZVU is too strong and prevents the EKF from following variations in
the height aiding sources, creating large innovations.

src/modules/ekf2/EKF/zero_velocity_update.cpp

@@ -51,7 +51,9 @@ void Ekf::controlZeroVelocityUpdate()
 			Vector3f vel_obs{0, 0, 0};
 			Vector3f innovation = _state.vel - vel_obs;
 
-			const float obs_var = sq(0.001f);
+			// Set a low variance initially for faster accel bias learning and higher
+			// later to let the states follow the measurements
+			const float obs_var = _NED_origin_initialised ? sq(0.2f) : sq(0.001f);
 			Vector3f innov_var{
 				P(4, 4) + obs_var,
 				P(5, 5) + obs_var,