added descend into navigation using existing land helper, downstream logic handles ignoring position setpoints (despite being populated), tested with multirotor in gazebo

Timestamps didn't exist when commander was run on the Linux side.
However, commander is now always run on the QURT side, so the hack is no
longer needed.

PWM output is now stopped if the mavlink actuator_controls stop being
sent. This means props will stop in case the Snapdragon crashes or the
cable from Snapdragon to Pixhawk/Pixracer breaks.

The mavlink instance USB can now be shutdown, so this hack is not needed
anymore.

This includes taking care of the arming state and the limit ramp.

This is by far the easiest solution for now.

This adds a custom passthrough "vehicle" to be used on
Pixhawk/Pixfalcon/Pixracer together with Snapdragon for RC input and PWM
output.

If this is not done, the aDSP crashes, not quite sure why but it's gotta
be something about the context of the callback.