enable automatic chromeos keymap generation under systemd

These three changes enable Chromebooks' automatic keyboard map generation to work on systemd similar to on OpenRC: https://wiki.postmarketos.org/wiki/Category:ChromeOS#Keyboard_mapping

main/cros-keyboard-map: add systemd service and supporting files

Using systemd-tmpfiles to create the directory in /tmp seems to be the most systemd way to do it. The config file for the systemd has a different format because the format of the openrc file doesn't lend itself to being used in a systemd service - compatibility would require a more complex ExecStart line or a wrapper script. I wasn't sure where to place the config file as /etc/conf.d doesn't seem to be used on systemd so I put it straight into /etc/. If this approach is merged I will update the wiki linked above.

I wanted to suggest what looks to me like the straightforward approach initially, but this can be changed if it's not satisfactory.

The other two changes are straightforward - creating an empty package to ensure this is installed on systemd cros devices and then enabling the service by default.