[subsystem] Make it possible to poweron certain subsystems trough LED

[fvantienen]

This is needed for certain boards that require it to power on the subsystem before using it.

[gautierhattenberger]

It would be better to use gpio interface here

[fvantienen]

Yeah will change that asap

[fvantienen]

Now changed it to use the gpio interface, like autopilot power.

[flixr]

Is gpio set always power on, or maybe clearing it is on in some cases? (since you can sink more power on a gpio to e.g. drive a FET)

[fvantienen]

I now changed it so that it is gpio_set by default, but you are able to define another function if needed.