Generate systemd-fsck-root.service in the initramfs
In the initrafms, generate a systemd-fsck-root.service to replace
systemd-fsck@<sysroot-device>.service. This way, after we transition
to the real root, systemd-fsck-root.service is marked as already done.
This introduces an unnecessary synchronization point, because
systemd-fsck@* is ordered after systemd-fsck-root also in the
initramfs. In practice this shouldn't be a problem.
https://bugzilla.redhat.com/show_bug.cgi?id=
1201979
C.f.
956eaf2b8d6c9999024705ddadc7393bc707de02.