ekf2: make filter update period configurable

- introduces new parameter EKF2_PREDICT_US to configure the filter
update period in microseconds
 - actual filter update period will be an integer multiple of IMU

src/modules/ekf2/EKF/covariance.cpp

@@ -56,7 +56,7 @@ void Ekf::initialiseCovariance()
 	_delta_angle_bias_var_accum.setZero();
 	_delta_vel_bias_var_accum.setZero();
 
-	const float dt = FILTER_UPDATE_PERIOD_S;
+	const float dt = _dt_ekf_avg;
 
 	resetQuatCov();
 
@@ -122,8 +122,8 @@ void Ekf::predictCovariance()
 	const float &dvz_b = _state.delta_vel_bias(2);
 
 	// Use average update interval to reduce accumulated covariance prediction errors due to small single frame dt values
-	const float dt = FILTER_UPDATE_PERIOD_S;
-	const float dt_inv = 1.0f / dt;
+	const float dt = _dt_ekf_avg;
+	const float dt_inv = 1.f / dt;
 
 	// convert rate of change of rate gyro bias (rad/s**2) as specified by the parameter to an expected change in delta angle (rad) since the last update
 	const float d_ang_bias_sig = dt * dt * math::constrain(_params.gyro_bias_p_noise, 0.0f, 1.0f);