sensor_mag.msg: add is_external flag & set it in the mag drivers

With this we don't have to use the ioctl MAGIOCGEXTERNAL, which does not
work on POSIX (eg. RPi).

src/drivers/ist8310/ist8310.cpp

@@ -913,6 +913,7 @@ IST8310::collect()
 
 	/* this should be fairly close to the end of the measurement, so the best approximation of the time */
 	new_report.timestamp = hrt_absolute_time();
+	new_report.is_external = !sensor_is_onboard;
 	new_report.error_count = perf_event_count(_comms_errors);
 	new_report.scaling = _range_scale;
 	new_report.device_id = _device_id.devid;