neonvm-kernel: kernel build improvements

[mikhail-sakhnov]

Fix readme file for kernel builder
Update ubuntu to LTS
Rename kernel config file

[sharnoff]

nice, thanks!

[Omrigan]

Closes #1154?

[Omrigan]

1. Should it be explained in this section that you need to do upgrade twice: once for x86, and once for arm64?
2. Would it be possible to do this:

export KERNEL_ARCH=...
export OLD_VERSION=...
export NEW_VERSION-...

in the first step, and then pass the variables to docker run with --env, and overall make it so the remaining commands don't need to be modified?

[mikhail-sakhnov]

Closes #1154?

@Omrigan No, it doesn't, it was originally a quick fix in the readme, later we decided with @sharnoff to rename files as well to have arch in sync, while renaming files I found that ubuntu version we use has repo ran out of the support, now I added your suggested improvals and hope that would be the last iteration of "fixing readme" 😃

UPD: it actually does, I originally looked into separate kernel issue, my fault.

[sharnoff]

If these environment variables are defined on the host, are they actually available inside the container?

[mikhail-sakhnov]

Oh, I passed them as a build-arg 🤦 Moved to the docker run -e

[mikhail-sakhnov]

Closing the PR because I accidentally rebased and pushed version from the other dev machine, and now some changes were losh 🤷 . Will re-open from the correct branch during normal working hours.