Use more general MAVLink commands rather than setting channel_overrides

[hamishwillee]

This uses a custom command to set the channel_override for camera aiming rather than using the "proper" API Vehicle.channel_override.

However the impression I get from @tridge is that we don't want to recommend using channel_overrides for this sort of thing at all! It is designed for simulating user input or things like implementing a joystick - not for direct control of the vehicle. In particular this is not a particularly good idea because the example relies on the channels being configured as you have done, and this could change.

Can you change this to use a command that abstracts the mount method? For example can MAV_CMD_DO_MOUNT_CONTROL be used to control the camera outside of missions - or possibly MAV_CMD_DO_MOUNT_CONFIGURE with the appropriate mount mode?

[mrpollo]

@arthurbenemann @geeksville what do yo guys think of this?