Generate systemd-fsck-root.service in the initramfs

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.