[Snap] Give add-ons access to GPIO

[benfrancis]

Some add-ons (e.g. the GPIO add-on) require access to GPIO ports.

Thus may require the gpio and/or gpio-control interfaces. I understand these interfaces are not usually connected by default.

[dilyn-corner]

One important thing to recognize about the GPIO interfaces is that they require a connection to the slot provider, and the system (snapd, core) are not the slot providers; the gadget snap is.

This means that the gadget snap needs to define the GPIO interfaces for your snap to connect to, see here for an example. This is true of most hardware interfaces (serial, pwm, i2c, etc).

And you are correct in saying these interfaces do not autoconnect by default. Generally, hardware interfaces are also super-privileged.