Plane: add aux channels options (ch5, ch6, ch7)

(please, give me time for make a good review of this pull request)

Hello everyone,
Now we can play with Tx switches for different uses while their output
channels are being busy in other tasks. For example, now you can use
input channel 5 for trigger a camera while channel 5 output is
configured for gimbal roll stabilization.

Regards from Spain,
Dario