This already has its own roadmap issue: #978

• Mutable overlay with special mode to allow us using the original directory as upperdir (blocker for systemd-confext in Flatcar)
• Setup overlay from the initrd to improve support for early boot configuration (udev, unit drop-ins, kernel modules or parameters, systemd-tmpfiles)
• WIP: Upstream daemon-reload (and service reload) support, drop our workarounds
• Upstream check-conflicts flag that prevents loading extensions when one extension would either shadow the base OS or another extension
• Wrapper around ignition-apply that provides two artifacts: one baked systemd-confext image and a slim Ignition config to use it when provisioning, maybe including a systemd-sysupdate config