Add Live ISO sources to Git

[lslezak]

Problem

The Live ISO sources are not maintained at GitHub, that makes the development complicated:

• Cannot track the changes easily, working with history in OBS is limited and more complicated than with Git
• Changing files in tarballs is difficult, you have to unpack it, modify a file and then manually compress back with correct parameters
• Building the Live ISO locally is not trivial

Solution

• Move the sources into the /live subdirectory
• Add GitHub action for automatic submission from master to the systemsmanagement:Agama:Staging OBS project
• Allow running the ISO build locally via make build

Changes

The individual files were not touched, I just copied them from the current OBS project. I only moved the fw_cleanup.rb script to the /tmp directory as the script is temporary and it is deleted at the end of the ISO build.

• All Kiwi tarballs were unpacked to Git
• Simple make builds the sources to the dist subdirectory and builds the tarballs
• The ISO can be built locally using make build
• Updated/extended the documentation in README.md (There are two README.md files, the main in /live which documents the details and the /live/src/README.md which is copied to OBS. Mostl importantly it says "do not touch this in OBS, use GitHub".)

Testing

• Tested manually
• The autosubmission works (see the GitHub action log)
• The built ISO works (tested in my agama-live branch project)
• Local build works as well

[imobachgs]

It looks great. Thanks!

It would be even better if we update (or remove) the following references:

• https://github.com/openSUSE/agama/blob/master/doc/live_iso.md (obsolete, right?)
• https://github.com/openSUSE/agama/blob/master/PACKAGING.md#the-live-iso

[imobachgs]

I guess we will need to drop this ALP reference at some point. But it is out of scope of this PR.

[kobliha]

Just a comment, ALP is dead

[lslezak]

As @imobachgs mentioned above, let's solve that separately later.

[kobliha]

ALP -> FO? Or call it ... call it something else?

[lslezak]

• Updated documentation. IMHO https://github.com/openSUSE/agama/blob/master/doc/live_iso.md still makes sense, I just added a link to the sources in Git.
• Updated the Makefile to use UTC for the time stamps in the tarballs, that should be more consistent and reproducible (the local time zone should not matter now).
• Removed IMAGE.md, that's actually now included in the live/README.md file.

[imobachgs]

Some Kiwi capitalization... otherwise, LGTM! (feel free to merge).