Ability to create a combustion-only ISO

[dgiebert]

Changed the behavior to allow for easy integration with SUMA builds.

1. Added an imageType named combustion
2. When using imageType: combustion removed the need to pass in a baseImage
3. Create a script that packages the combustion and artifacts folder into an ISO
4. Since in plain combustion the mount is done automatically remove the mount

[rdoxenham]

Thanks so much for the comprehensive contribution @dgiebert - can you help me understand this a little bit? Looks like you're using EIB to generate just the combustion directory and generating a tarball? Can you please expand on this and perhaps also add some example documentation to your commit? Thanks again.

[dgiebert]

Yes, for our Hackweek project to combine EIB with SUMA/Kiwi, we had the idea to reverse the image creation process.

1. Create the Combustion + Artefact directory
2. Build the custom image using SUMA

This has the advantage of having all the benefits of Kiwi, only building once, and an easier way to enable frozen patch levels.