Fold patchset processing into tarball service

[pawiecz]

but why not extend the tarball service so it can generate checkout nodes with configured patchset(s) applied?

As it turns out, kbuild nodes currently listen for checkout nodes, so I'm fearing that adding another type of basically the same thing (i.e. provide a kernel source tree) could introduce unneeded complexity.

This would of course need some thinking around how to configure patchsets in pipeline.yaml, but it feels more intuitive to me (remember I don't have any background information/history wrt. this feature, though)

Originally posted by @a-wai in #342 (comment)