[Plugin] FollowMe mode Validation

[shakthi-prashanth-m]

While implementing FollowMe plugin for DroneCore,

• I changed flight_mode to Follow Me usng MAV_CMD_DO_SET_MODE; It worked.
• But after that, I am emitting motion reports periodically with interval of 1 second by faking hard-coded GPS info. I am able to see these are received by PX4 MAVLink receiver (see below)

INFO  [uorb]  uORB::DeviceNode::publish: Success
INFO  [mavlink] Follow Target lat: 473977448, long: 85455953, alt 530.040039
INFO  [mavlink] INFO  [uorb]  uORB::DeviceNode::publish: Success
INFO  [mavlink] Follow Target lat: 473977458, long: 85455958, alt 540.040039
INFO  [mavlink] INFO  [uorb]  uORB::DeviceNode::publish: Success
INFO  [mavlink] Follow Target lat: 473977468, long: 85455963, alt 550.040039
INFO  [mavlink] INFO  [uorb]  uORB::DeviceNode::publish: Success
INFO  [mavlink] Follow Target lat: 473977478, long: 85455968, alt 560.040039
INFO  [mavlink] INFO  [uorb]  uORB::DeviceNode::publish: Success
INFO  [mavlink] Follow Target lat: 473977488, long: 85455973, alt 570.040039
INFO  [mavlink] INFO  [uorb]  uORB::DeviceNode::publish: Success
INFO  [mavlink] Follow Target lat: 473977498, long: 85455978, alt 580.040039

But, I don't see them reflecting in the Vehicle (Gazebo).
Am I missing something ?

Thanks

Goes with #142

[shakthi-prashanth-m]

Added support in firmware & merged. Thanks @julianoes @LorenzMeier