mkinitcpio: use systemd and adapt /etc/fstab

[NeroReflex]

Use systemd on initrd instead of busybox to solve two bugs:

• when two chimeraos are installed booting a device might result in mounting the rootfs of the other device
• Issue #919 https://github.com/ChimeraOS/chimeraos/issues/919

/efi will be automounted by systemd: remove the fstab entry.

If the default subvolume of / is changed boot will fail: fix this by mentioning subvolumes by theirs ID (fixed since installation).

This superceedes https://github.com/ChimeraOS/chimeraos/pull/901 and https://github.com/ChimeraOS/chimeraos/pull/914