Updates to the Kodi role and a little refactoring.

- Using the GBM backend for Kodi failed (maybe it's because I'm using
Flatpak, maybe not). Weston and Wayland failed (maybe the Weston version
in Buster is too old). Go back to good old Xorg.
- Configure Plymouth boot spalsh.
- Refactor GRUB and initramfs to the common debian_server role.
- Disable cgroup2 (unified hierarchy) because Docker in Buster doesn't
support it (Bullseye does) and with buster-backports it's now enabled by
default.