Add comments for which command line options override which variables

(Useful for me as I just reading the script from the top and editing these values, without realising I didn't have to.)
mmc-install.sh/sata-install.sh: Pass through additional args (ie -m) to novena-image
Add a temporary repo.novena.io entry in sources.list.d by default, allowing Novena-specific packages in -a

Can be disabled with -n/--no-novena-repo or customised with --novena-repo
Allow cross-building a Novena image from a non-ARM host, using qemu-user