Unify shallow clone options across rootfs scripts to reduce traffic and storage requirements

> Are clone options intended to reduce traffic or do they serve some additional purpose?

Yes, to help reduce network traffic and speed up clones in CI systems, where complete history is not required.

_Originally posted by @vigneshraman in https://github.com/kernelci/kernelci-core/pull/2826#discussion_r2120894107_

Rootfs builder scripts use various approaches to running shallow clones. This task focuses on providing a set of sane defaults for such an operation.

- [x] Check `git clone`s across rootfs scripts
- [ ] Extract optimum set of git options
- [ ] Document it as a best practice
- [ ] (Optionally) Provide it as a shell function shared across scripts

Review the git clone commands across the rootfs builder scripts, then compare their shallow-clone options and CI requirements. Determine a consistent set of defaults, document the best practice, and consider whether a shared shell function is appropriate; done means the scripts use an agreed approach and the guidance is recorded.

Written by the indexing model from the issue text.