fix(sys-kernel/coreos-kernel): Add evdev module.

ACPI power buttons are input devices! Without this it isn't possible to
trigger a graceful shutdown via qemu's command 'system_powerdown' or
whatever libvirt and similar APIs that are layered on top of it.

Probably applicable to other things too that we just don't know about.
Merge pull request #144 from marineam/evdev

fix(sys-kernel/coreos-kernel): Add evdev module.