This makes the hook ok to use cross-flavor.
We could also move glib-compile-schemas to a separate hook, to ensure we never
silently fail because glib-compile-schemas is broken/missing.
When the files we're creating in the live layer have static content, ship
them in live-build/ubuntu/includes.chroot.minimal.standard.live instead of
generating them from live-build/ubuntu/hooks/020-ubuntu-live.chroot_early.
Also fixes the fact that
live-build/ubuntu/hooks/020-ubuntu-live.chroot_early was incorrectly writing
to /root in the previous upload instead of /usr.