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/platforms/posix/drivers/accelsim/accelsim.cpp

@@ -915,8 +915,7 @@ ACCELSIM::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);
@@ -946,6 +945,7 @@ ACCELSIM::mag_measure()
 
 
 	mag_report.timestamp = hrt_absolute_time();
+	mag_report.is_external = false;
 
 	mag_report.x_raw = (int16_t)(raw_mag_report.x / _mag_range_scale);
 	mag_report.y_raw = (int16_t)(raw_mag_report.y / _mag_range_scale);

src/platforms/posix/drivers/df_ak8963_wrapper/df_ak8963_wrapper.cpp

@@ -278,6 +278,7 @@ int DfAK8963Wrapper::_publish(struct mag_sensor_data &data)
 
 	mag_report mag_report = {};
 	mag_report.timestamp = hrt_absolute_time();
+	mag_report.is_external = true;
 
 	// TODO: remove these (or get the values)
 	mag_report.x_raw = 0;

src/platforms/posix/drivers/df_hmc5883_wrapper/df_hmc5883_wrapper.cpp

@@ -269,6 +269,7 @@ int DfHmc9250Wrapper::_publish(struct mag_sensor_data &data)
 
 	mag_report mag_report = {};
 	mag_report.timestamp = hrt_absolute_time();
+	mag_report.is_external = true;
 
 	/* The standard external mag by 3DR has x pointing to the
 	 * right, y pointing backwards, and z down, therefore switch x

src/platforms/posix/drivers/df_lsm9ds1_wrapper/df_lsm9ds1_wrapper.cpp

@@ -624,6 +624,7 @@ int DfLsm9ds1Wrapper::_publish(struct imu_sensor_data &data)
 
 	accel_report.timestamp = gyro_report.timestamp = hrt_absolute_time();
 	mag_report.timestamp = accel_report.timestamp;
+	mag_report.is_external = false;
 
 	// TODO: get these right
 	gyro_report.scaling = -1.0f;

src/platforms/posix/drivers/df_mpu9250_wrapper/df_mpu9250_wrapper.cpp

@@ -700,6 +700,7 @@ int DfMpu9250Wrapper::_publish(struct imu_sensor_data &data)
 
 	if (_mag_enabled) {
 		mag_report.timestamp = accel_report.timestamp;
+		mag_report.is_external = false;
 
 		mag_report.scaling = -1.0f;
 		mag_report.range_ga = -1.0f;