ostree-prepare-root.service: Run earlier in initrd

ostree-prepare-root.service: Run earlier in initrd

Previously, we were preparing the root very late in the boot process;
right before we switch root. The issue with that is that most services
in the initrd that run `After=initrd-root-fs.target` expect that
`/sysroot` already points to the rootfs we'll be pivoting to. Running
this late violates that assumption.

This patch fixes this by making `ostree-prepare-root.service` instead
run right after `sysroot.mount` (the physical sysroot mounted by
systemd) but still before `initrd-root-fs.target` (which is the target
signalling that `/sysroot` is now valid and ready).

This should make it easier to integrate OSTree with other initrd
services such as Ignition.

Related: https://github.com/dustymabe/ignition-dracut/issues/20

Closes: #1759
Approved by: cgwalters