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/bmm150/bmm150.cpp

@@ -251,7 +251,6 @@ BMM150 :: BMM150(int bus, const char *path, bool external, enum Rotation rotatio
 	_external(false),
 	_running(false),
 	_call_interval(0),
-	_report{0},
 	_reports(nullptr),
 	_collect_phase(false),
 	_scale{},
@@ -344,7 +343,7 @@ int BMM150::init()
 	}
 
 	/* allocate basic report buffers */
-	_reports = new ringbuffer::RingBuffer(2, sizeof(_report));
+	_reports = new ringbuffer::RingBuffer(2, sizeof(mag_report));
 
 	if (_reports == nullptr) {
 		goto out;
@@ -433,7 +432,7 @@ BMM150::stop()
 ssize_t
 BMM150::read(struct file *filp, char *buffer, size_t buflen)
 {
-	unsigned count = buflen / sizeof(_report);
+	unsigned count = buflen / sizeof(mag_report);
 	struct mag_report *mag_buf = reinterpret_cast<struct mag_report *>(buffer);
 	int ret = 0;
 
@@ -684,6 +683,7 @@ BMM150::collect()
 
 
 	mrb.timestamp = hrt_absolute_time();
+	mrb.is_external = is_external();
 
 	// report the error count as the number of bad transfers.
 	// This allows the higher level code to decide if it

src/drivers/bmm150/bmm150.hpp

@@ -228,7 +228,6 @@ private:
 	unsigned        _call_interval;
 
 
-	struct mag_report _report;
 	ringbuffer::RingBuffer  *_reports;
 
 	bool            _collect_phase;

src/drivers/hmc5883/hmc5883.cpp

@@ -997,6 +997,7 @@ HMC5883::collect()
 	// XXX revisit for SPI part, might require a bus type IOCTL
 	unsigned dummy;
 	sensor_is_onboard = !_interface->ioctl(MAGIOCGEXTERNAL, dummy);
+	new_report.is_external = !sensor_is_onboard;
 
 	if (sensor_is_onboard) {
 		// convert onboard so it matches offboard for the

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;

src/drivers/lis3mdl/lis3mdl.cpp

@@ -945,6 +945,7 @@ LIS3MDL::collect()
 
 	unsigned dummy;
 	sensor_is_onboard = !_interface->ioctl(MAGIOCGEXTERNAL, dummy);
+	new_report.is_external = !sensor_is_onboard;
 
 	/*
 	 * RAW outputs

src/drivers/lsm303d/lsm303d.cpp

@@ -1653,8 +1653,7 @@ LSM303D::mag_measure()
 	} raw_mag_report;
 #pragma pack(pop)
 
-	mag_report mag_report;
-	memset(&mag_report, 0, sizeof(mag_report));
+	mag_report mag_report {};
 
 	/* start the performance counter */
 	perf_begin(_mag_sample_perf);
@@ -1680,6 +1679,7 @@ LSM303D::mag_measure()
 	 */
 
 	mag_report.timestamp = hrt_absolute_time();
+	mag_report.is_external = is_external();
 
 	mag_report.x_raw = raw_mag_report.x;
 	mag_report.y_raw = raw_mag_report.y;

src/drivers/mpu9250/mag.cpp

@@ -205,6 +205,7 @@ MPU9250_mag::_measure(struct ak8963_regs data)
 
 	mag_report	mrb;
 	mrb.timestamp = hrt_absolute_time();
+	mrb.is_external = false;
 
 	/*
 	 * Align axes - note the accel & gryo are also re-aligned so this