[WIP] UAP_HAL_Linux: Basic safety switch infrastructure

[landswellsong]

Allows Linux boards to define a GPIO pin to be treated as a safety switch. Current implementation assumes an active low disarm with the pin being pulled up (via non-ardupilot means, such as device trees or physically on the PCB etc).

The code isn't directly used anywhere as I test it on a WIP internal branch for now, but it is indeed operational, e.g. shorting the pin to the ground correctly sets a disarmed (1) status:

NOTE: The virtual destructor added deep in the GPIO classes hierarchy not to be locked into a particular flavor of Linux board GPIO. I'm honestly unsure why it wasn't there in first place since GPIO::channel does instantiate an object which needs to be eventually freed using a base class pointer. I checked another instances of this method's usage and it seems that the pointer is never freed. So it's either a virtual destructor is indeed needed or Linux sysfs GPIO class needs to be changed in order to track the pointers that are generated by the channel() method.

[landswellsong]

Actually I learned about force_safety_XX methods so I'll be adding those here too and then remove the WIP tag.