EKF: add baro bias estimator using GNSS altitude

src/modules/ekf2/EKF/common.h

@@ -257,6 +257,7 @@ struct parameters {
 	float gps_pos_noise{0.5f};		///< minimum allowed observation noise for gps position fusion (m)
 	float pos_noaid_noise{10.0f};		///< observation noise for non-aiding position fusion (m)
 	float baro_noise{2.0f};			///< observation noise for barometric height fusion (m)
+	float baro_drift_rate{0.005f}; ///< process noise for barometric height bias estimation (m/s)
 	float baro_innov_gate{5.0f};		///< barometric and GPS height innovation consistency gate size (STD)
 	float gps_pos_innov_gate{5.0f};		///< GPS horizontal position innovation consistency gate size (STD)
 	float gps_vel_innov_gate{5.0f};		///< GPS velocity innovation consistency gate size (STD)