Persistent memory support for hybrid ISOs

[Nowa-Ammerlaan]

This adds a new option for use in the spec files which will toggle the creation of an extra partition in the hybrid ISO image. The partition file system is XFS only (for now), and is intended for use as persistent storage when the ISO is dded to e.g. an USB drive. The extra partition has no effect when the ISO is burned to an optical medium as partitioning and writing back is not really supported there.

There are a couple of things still to discuss (either now or in a follow-up PR):

• Do we want the file system type to be variable, i.e. add another setting so we can choose something other then XFS?
• Can we change the name of the partition (currently Appended3)? Do we want to? The xorriso manual is unclear where the name comes from.
• Do we want to automatically mount this partition at boot (i.e. add it to the fstab)? If so where should we mount it?

See-also: https://gist.github.com/immolo/16de4fe7907991878585eb2c56e31f27

CC @immolo

[immolo]

This is awesome, thank you <3 I have test images I can share in IRC however it works extremely well in the tests I have run.

Can we change the name of the partition?

Inside the DEs I have tested it just shows up $PART-SIZE as the name to users and it's as good as name as any to use a LABEL name for mounting. I vote leave it as it is.

Do we want to automatically mount?

Seems logically, I'm currently using fsscript to do the same task. As for name /mnt/storage?

[Nowa-Ammerlaan]

Seems logically, I'm currently using fsscript to do the same task. As for name /mnt/storage?

Done! I made the mounting point an argument for the script, so we can more easily turn it into a setting for the spec file if we want to in the future.

[akhuettel]

This looks great. Pulling it in - and sorry for the long waiting time. In case of doubt poke me in IRC in the future.