Start 2nd MAVLink instance in Gazebo SITL

posix-configs/SITL/init/rcS_gazebo_iris

@@ -50,6 +50,7 @@ mc_pos_control start
 mc_att_control start
 mixer load /dev/pwm_output0 ../../../../ROMFS/px4fmu_common/mixers/quad_w.main.mix
 mavlink start -u 14556 -r 2000000
+mavlink start -u 14557 -r 2000000 -m onboard
 mavlink stream -r 80 -s POSITION_TARGET_LOCAL_NED -u 14556
 mavlink stream -r 80 -s LOCAL_POSITION_NED -u 14556
 mavlink stream -r 80 -s GLOBAL_POSITION_INT -u 14556