Sysext Build refactoring

[t-lo]

This change refactors the build process, adds library functions for common tasks,

This is a work in progress.

Done:

• New framework for writing sysext generators
• bakery.sh central entry point
• sysext creation scripts only implement function stubs
• library implements sysext image generation (replacing bake.sh)
• skeleton directory available to derive new sysext generators from
• all sysexts can now print upstream release versions available
• almost sysext generators converted.
• Readme updated; links to sysext docs fixed (now works from REDME.md and from index.md)
• tooling for common maintenance tasks added (e.g. for in-depth comparing sysexts)
• rebuild and compare all sysexts currently published

TODO:

• adjust release-build.sh to the new build process
• Use new upstream release version print feature in release-build.sh

Oh, and I added a nerdctl sysext.

Thanks to @jmacdonagh for help with the Tailscale sysext; I added most of the changes from #105.

How to test

• clone the repo, check out the branch
• List sysexts, list upstream release versions, and build sysexts as documented in https://github.com/flatcar/sysext-bakery/blob/t-lo/scripts-refactoring/docs/index.md#working-with-the-bakery
• Download sysexts from the bakery release, try rebuilding locally, and compare using the tools/compare.sh script.

Testing done

• Build all sysexts in at least one version for x86-64 and compared with at least one release from the bakery.
  • sometimes (rarely) binaries differ, I guess these get re-published? Config files and directory layouts always match.
• tools/ will automate rebuilding all releases and comparing.