restructure kernel builds into directories

[deitch]

- What I did

Restructured the kernel/ directory so that each kernel series and variant has its own directory with config files and patches, rather than a mass of them in the same directory.

And documented it, of course.

- How I did it

Made directories, moved and renamed files, updated the Dockerfile, updated the docs.

- How to verify it

Build it locally. I did.

- Description for the changelog

Restructured kernel build directories.