ekf2: Improve control of magnetometer bias learning

Adds parameters so that the motion checks used to switch between magnetic yaw and 3-axis fusion can be adjusted.
Modifies the check used to determine if learned mag biases  can be saved. A cumulated calibration time is used rather than continuous calibration time to allow for for switching in and out of 3-axis fusion mode that is required to do calibration.

src/modules/ekf2/ekf2_params.c

@@ -447,7 +447,7 @@ PARAM_DEFINE_INT32(EKF2_DECL_TYPE, 7);
  * Type of magnetometer fusion
  *
  * Integer controlling the type of magnetometer fusion used - magnetic heading or 3-axis magnetometer.
- * If set to automatic: heading fusion on-ground and 3-axis fusion in-flight
+ * If set to automatic: heading fusion on-ground and 3-axis fusion in-flight with fallback to heading fusion if there is insufficient motion to make yaw or mag biases observable.
  *
  * @group EKF2
  * @value 0 Automatic
@@ -457,6 +457,30 @@ PARAM_DEFINE_INT32(EKF2_DECL_TYPE, 7);
  */
 PARAM_DEFINE_INT32(EKF2_MAG_TYPE, 0);
 
+/**
+ * Horizontal acceleration threshold used by automatic selection of magnetometer fusion method.
+ * This parameter is used when the magnetometer fusion method is set automatically (EKF2_MAG_TYPE = 0). If the filtered horizontal acceleration is greater than this parameter value, then the EKF will use 3-axis magnetomer fusion.
+ *
+ * @group EKF2
+ * @min 0.0
+ * @max 5.0
+ * @unit m/s**2
+ * @decimal 2
+ */
+PARAM_DEFINE_FLOAT(EKF2_MAG_ACCLIM, 0.5f);
+
+/**
+ * Yaw rate threshold used by automatic selection of magnetometer fusion method.
+ * This parameter is used when the magnetometer fusion method is set automatically (EKF2_MAG_TYPE = 0). If the filtered yaw rate is greater than this parameter value, then the EKF will use 3-axis magnetomer fusion.
+ *
+ * @group EKF2
+ * @min 0.0
+ * @max 0.5
+ * @unit rad/s
+ * @decimal 2
+ */
+PARAM_DEFINE_FLOAT(EKF2_MAG_YAWLIM, 0.25f);
+
 /**
  * Gate size for barometric height fusion
  *