gadget snap should allow a gpio's direction to be set

[Tony Espy]

The gadget snap should allow the direction of a gpio to be initialized in the gadget snap. Currently, when a gpio slot is connected, a systemd unit is created which exports the gpio, which could easily be extended to also set the direction.

For instance, here's an existing gadget snap gpio definition:

slots:
    cloud-led:
        interface: gpio
        number: 346

This instead would become:

slots:
    cloud-led:
        interface: gpio
        number: 346
        direction: out

This would control the value of the following sysfs node:

/sys/class/gpio/gpio346/direction

This is being reported on an Ubuntu Core 16 system running snapd 2.25 from core snap 16-2 (r1804).