sensors: always start baro/GPS/mag aggregators if SYS_HAS_* set

- add new SYS_HAS_GPS parameter

src/lib/systemlib/system_params.c

@@ -191,6 +191,20 @@ PARAM_DEFINE_INT32(SYS_CAL_TMIN, 5);
  */
 PARAM_DEFINE_INT32(SYS_CAL_TMAX, 10);
 
+/**
+ * Control if the vehicle has a GPS
+ *
+ * Disable this if the system has no GPS.
+ * If disabled, the sensors hub will not process sensor_gps,
+ * and GPS will not be available for the rest of the system.
+ *
+ * @boolean
+ * @reboot_required true
+ *
+ * @group System
+ */
+PARAM_DEFINE_INT32(SYS_HAS_GPS, 1);
+
 /**
  * Control if the vehicle has a magnetometer
  *