cork()/push() will be called in the vehicle code.
The issue that led to this fix was as follows:
- on a Linux board (Navio 2) usage of this class didn't yield any PWM on
the expected outputs on Rover (Copter and Plane worked as expected)
After dicsussion at #6339 rover code was changed (541538f) to use make
proper use of SRV_Channels API.
This fix is also the consequence of the discussion. TL;DR: the output is
only done when commanded by the vehicle.
Georgii Staroselskii
Andrew Tridgell
Mathieu OTHACEHE
José Roberto de Souza