ostree-prepare-root: Refactor: Create /sysroot.tmp much later

ostree-prepare-root: Refactor: Create /sysroot.tmp much later

Typically we have our ready made-up up root at
`/sysroot/ostree/deploy/.../` (`deploy_path`) and the real rootfs at
`/sysroot` (`root_mountpoint`).  We want to end up with our made-up root
at `/sysroot/` and the real rootfs under `/sysroot/sysroot` as systemd
will be responsible for moving `/sysroot` to `/`.

We need to do this in 3 moves to avoid trying to move `/sysroot` under
itself:

1. `/sysroot/ostree/deploy/...` -> `/sysroot.tmp`
2. `/sysroot` -> `/sysroot.tmp/sysroot`
3. `/sysroot.tmp` -> `/sysroot`

This is a refactoring to group all these operations together so I can
implement an alternative in terms of `pivot_root`.

Closes: #403
Approved by: cgwalters