main/postmarketos-mkinitfs: Correctly set up udev

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.

Possibly fixes #1411 (comment 895465028).

To test, run

pmbootstrap install --fde --add unl0kr
pmbootstrap qemu

and verify that keyboard / mouse input works in unl0kr.

Background

The previous udevadm test workaround worked for most but apparently not all people. The test command is also explicitly marked as a debug tool, not meant to be used in production. Using the debug shell, I noticed that even after running udevadm test, libinput list-devices gave no output and libinput debug-events /dev/input/event... failed, claiming that the device had not been set up by udev yet.

Researching how we set up udev after the initfs phase, I found that the setup-udev script starts a number of RC services that combine udevd, udevadm trigger and udevadm settle.

After calling this series of commands in the debug shell, both libinput list-devices and libinput debug-events started working as expected.