main/postmarketos-base-ui: setup cgroups on boot (MR 4585)

Allows things like podman/docker to work out of the box.

I looked at /etc/init.d/cgroups, and having this service enabled isn't a
problem if the kernel doesn't have cgroup support... in that situation,
the service script just simply exits (success).

[ci:skip-build] already built successfully in CI