Put squashfs-label patch overlays in a separate file

The util-linux patch in particular causes a lot of rebuilds / cache
misses (linux kernel, llvm and hence clang, rustc, & ghc, etc.) in the
guest.

If the host is also building large stuff like the kernel locally for
other reasons, this can result in two sets of large, local builds. :(

Users sensitive to this may wish to harmonize the host and guest config
to get back down to one set of large local builds.  So we make the
squashfs-label patches easy to also apply to the host, for those that
wish to do so.