ekf2: improve attitude estimation without horizontal aiding

- fake_pos only if at rest or tilt variances becomes large
 - fake pos: don't run when grav fusion is enabled
 - gravity fusion enabled by default
 - gravity: only fuse when accel norm and lpf norm are consistent

Co-authored-by: bresch <brescianimathieu@gmail.com>

src/modules/ekf2/EKF/control.cpp

@@ -68,11 +68,9 @@ void Ekf::controlFusionModes(const imuSample &imu_delayed)
 	// monitor the tilt alignment
 	if (!_control_status.flags.tilt_align) {
 		// whilst we are aligning the tilt, monitor the variances
-		const Vector3f angle_err_var_vec = getQuaternionVariance();
-
-		// Once the tilt variances have reduced to equivalent of 3deg uncertainty
+		// Once the tilt variances have reduced to equivalent of 3 deg uncertainty
 		// and declare the tilt alignment complete
-		if ((angle_err_var_vec(0) + angle_err_var_vec(1)) < sq(math::radians(3.0f))) {
+		if (getTiltVariance() < sq(math::radians(3.f))) {
 			_control_status.flags.tilt_align = true;
 
 			// send alignment status message to the console