finalize-staged: Use the core option parsing to load sysroot
finalize-staged: Use the core option parsing to load sysroot

Prep for using the default mount namespace handling there that
will land as part of the read-only `/sysroot` and `/boot` work.
See https://github.com/ostreedev/ostree/issues/1265

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree