Implement the external mag param in a fashion that retains backward compatibility

src/modules/sensors/sensor_params.c

@@ -292,6 +292,19 @@ PARAM_DEFINE_FLOAT(SENS_BOARD_Z_OFF, 0.0f);
  */
 PARAM_DEFINE_INT32(SENS_EXT_MAG_ROT, 0);
 
+/**
+* Set usage of external magnetometer
+*
+*  * Set to 0 (default) to auto-detect (will try to get the external as primary)
+*  * Set to 1 to force the external magnetometer as primary
+*  * Set to 2 to force the internal magnetometer as primary
+*
+* @min 0
+* @max 2
+* @group Sensor Calibration
+*/
+PARAM_DEFINE_INT32(SENS_EXT_MAG, 0);
+
 
 /**
  * RC Channel 1 Minimum

src/modules/systemlib/system_params.c

@@ -82,14 +82,3 @@ PARAM_DEFINE_INT32(SYS_USE_IO, 1);
 * @group System
 */
 PARAM_DEFINE_INT32(SYS_RESTART_TYPE, 2);
-
-/**
-* Set usage of external magnetometer
-*
-* Set to 1 to use external magnetometer instead of internal one.
-*
-* @min 0
-* @max 1
-* @group System
-*/
-PARAM_DEFINE_INT32(SYS_EXT_MAG, 0);