- now we've a single `.git` for all kernels with all branches; a "master" cache
- back to full stable git bundle usage from kernel.org
- bye bye, "gitballs"
- split kernel-related git stuff into `kernel-git.sh`
- introduce `ARMBIAN_MOUNTPOINTS_DEPRECATED` @TODO actually use for cleaning non-Docker
- add `axel` hostdep for multi-conn download of bundle, which comes from a CDN.
- deprecate old mountpoints for gitballs and linux-kernel
- move cleaning of old sources/kernel to mountpoints code
