fix(overlayfs): split overlayfs mount in two steps

fix(overlayfs): split overlayfs mount in two steps

This commit splits the creation of required overlayfs underlaying
directories and the actual overlayfs mount. This way it is still
possible to mount the overlayfs with the generated sysroot.mount that
dmsquash-live creates.

The overlayfs tree is created in a pre-mount hook so it is executed
before sysroot.mount is started. Otherwise sysroot.mount starts and
fails before mount hooks are executed.

Signed-off-by: David Cassany <dcassany@suse.com>