prepare-root: avoid double-stacked /sysroot mount

prepare-root: avoid double-stacked /sysroot mount

prepare-root works with the mount that has been set up at /sysroot.
It creates a bind-mount within /sysroot (the deployment) and then moves
that mount to /sysroot.

Now we have 2 mounts both at /sysroot, and once we do switch_root, we will
never be able to unmount both of them. I'm not sure if this is ultimately
a kernel bug, but either way, ostree could do a bit more tidying up
after itself.
http://thread.gmane.org/gmane.linux.file-systems/92411

Easy way to reproduce:
1. Boot with rd.break param
2. At initramfs shell, run: ostree-prepare-root /sysroot
3. Observe two /sysroot mounts in /proc/mounts

Fix this by setting up the mounts at /sysroot.tmp, and unmounting the
original /sysroot before our new mount is MS_MOVEd on top of it.