Refactor rootfs.sh

[jodh-intel]

All the rootfs_lib.sh scripts use a repository URL. That could be a required variable in config.sh which would allow rootfs_lib.sh to shrink.

In fact, looking at rootfs_lib.sh, we should move the following to a common scripts/lib.sh that all the rootfs_lib.sh files can source:

• check_program()
• check_root()

Even generate_dnf() could be parameterised to allow that too to be moved to scripts/lib.sh.

The ideal would be that each repo would only need to specify:

• a config.sh

Iff rootfs_lib.sh exists for a distro, it could be used. But the default would be for rootfs.sh to call build_rootfs() from scripts/lib.sh by default (if there is no rootfs_lib.sh` for a particular distro).

The ideal would be to only have a single config file for all distros ("declarative") with all the logic in scripts/lib.sh.

[jodh-intel]

WDYT @jcvenegas?

[jcvenegas]

@jodh-intel I like the idea, I can work latter this week or next to have refactor it.

[jodh-intel]

Thanks @jcvenegas!

[jodh-intel]

It would be great to have this as it would make adding a new distro much simpler and would minimise code getting out of sync.