ekf2: enable multiple height sources fusion

Instead of having a single height source fused into the EKF and the
other ones "waiting" for a failure or the primary sensor, fuse all
sources in EKF2 at the same time. To prevent the sources from fighting against each
other, the "primary" source is set as reference and the other ones are
running a bias estimator in order to make all the secondary height
sources converge to the primary one.

If the reference isn't available, another one is automatically selected
from a priority list. This secondary reference keeps its current bias
estimate but stops updating it in order to be the new reference as close
as possible to the primary one.

src/modules/ekf2/EKF2.cpp

@@ -106,7 +106,7 @@ EKF2::EKF2(bool multi_mode, const px4::wq_config_t &config, bool replay_mode):
 	_param_ekf2_req_hdrift(_params->req_hdrift),
 	_param_ekf2_req_vdrift(_params->req_vdrift),
 	_param_ekf2_aid_mask(_params->fusion_mode),
-	_param_ekf2_hgt_mode(_params->vdist_sensor_type),
+	_param_ekf2_hgt_mode(_params->height_sensor_ref),
 	_param_ekf2_terr_mask(_params->terrain_fusion_mode),
 	_param_ekf2_noaid_tout(_params->valid_timeout_max),
 	_param_ekf2_rng_noise(_params->range_noise),
@@ -292,7 +292,7 @@ void EKF2::Run()
 		}
 
 		// if using baro ensure sensor interval minimum is sufficient to accommodate system averaged baro output
-		if (_params->vdist_sensor_type == 0) {
+		if (_params->fusion_mode & SensorFusionMask::USE_BARO_HGT) {
 			float sens_baro_rate = 0.f;
 
 			if (param_get(param_find("SENS_BARO_RATE"), &sens_baro_rate) == PX4_OK) {