Moar refactoring of CI scripts

I am working on #1759 (closed) and this is another PR with some small improvements and refactoring of Makefiles that will in the end enable implementation of #1700 (closed):

• few small simplifications and refactoring (see commit messages)
• gomod download stage does not require CI image
• add a stage that checks if generated files are in sync with those committed in the repo.

I think the last one is esp. important as this makes sure that git repository contents are always in sync with what would be generated by controller-gen. Two examples where both got desynchronized:

• !1671 (merged)
• !1656 (merged)