system_params: add SYS_HAS_MAG and SYS_HAS_BARO params

This allows to use PX4 on systems that do not have a mag or a baro,
such as the Omnibus F4 SD.

src/modules/sensors/parameters.cpp

@@ -188,6 +188,9 @@ void initialize_parameter_handles(ParameterHandles &parameter_handles)
 	(void)param_find("SYS_CAL_TDEL");
 	(void)param_find("SYS_CAL_TMAX");
 	(void)param_find("SYS_CAL_TMIN");
+
+	(void)param_find("SYS_HAS_MAG");
+	(void)param_find("SYS_HAS_BARO");
 }
 
 int update_parameters(const ParameterHandles &parameter_handles, Parameters &parameters)