vtol_att_control: ack transition commands

[julianoes]

Previously transition commands were not acked at all which meant that a
mavlink consumer such as a ground station would not get feedback if the
command arrived.

This solution is not optimal because it does not take into account if
the transition actually happened but at least it is feedback that the
command has arrived at the destination.

This solves an issue raised in mavlink/MAVSDK#150.

Tested in SITL using

make posix gazebo_standard_vtol

and for DroneCore:

make && build/default/integration_tests_runner --gtest_filter="SitlTest.ActionSimpleTransition"

[dagar]

Thanks, I was actually about to add this for QGC. mavlink/qgroundcontrol#5827

[julianoes]

@dagar do we need to check stack usage on NuttX?

[dagar]

Worst case it will dip into the 300 byte warning level and briefly be annoying.