Add mag bias estimator to CAN GPS units with IMUs

- run `sensors` hub to process sensor_mag and publish vehicle_magnetometer
 - update uavcannode to use vehicle_magnetometer

ROMFS/cannode/init.d/rcS

@@ -96,6 +96,17 @@ then
 	gyro_calibration start
 fi
 
+
+if param compare -s MBE_ENABLE 1
+then
+	# conservative mag bias estimation
+	param set-default MBE_LEARN_GAIN 5
+	param set-default IMU_GYRO_CUTOFF 20
+	mag_bias_estimator start
+fi
+
+param set-default SENS_MAG_RATE 100
+
 sensors start
 
 uavcannode start