This replaces the previous `udevadm test` hack with a combination of
`udevd`, `udevadm trigger` and `udevadm settle` in order to coldplug all
available devices and make them available for use via libinput / unl0kr.
Relates to: #1411
[ci:skip-build] already built successfully in CI
This copies rules from /etc/udev/rules.d as well which ensures that
device-specific udev rules (e.g. for Nokia N900) are available in the
initfs.
Relates to: #1411
