[arch-releng] [RFC] Change from gzip to xz in SquashFS images

Now that Linux 2.6.38 hits [core] we can enable xz compression in SquashFS images by default. I can not test it now, but is supposed to work fine. Go ahead and apply to master? https://github.com/djgera/archiso/compare/ce31b12654...4ea8b93f9c