proposal: Do not depend on systemd-sysext

[tulilirockz]

Since systemd-sysext is just glorified overlayfs, we could not depend on it by using overlayFS to mount everything up on the target system (the best idea would be to have a tag for enabling/disabling systemd-sysext). This would increase compatibility to quite literally any system that has OverlayFS support, even stuff like the BSD family could have support for this