Setup systemd-sysext for the rootfs in the initrd already

[pothos]

Run systemd-sysext for /sysroot in the initrd already, and make systemd-sysext.service handle this (first upstream PR systemd/systemd#27161 but ideally the refresh would be skipped), this can be tested in Flatcar (should run after initrd-setup-root-after-ignition.service) but this new initrd unit should be upstreamed.

As far as I know the upstream wish was also to support mounting /var through a kernel cmdline parameter to be able to account for sysext images under /var but not sure if this is a blocker.