Integrate stub generation into cocotb build flows

[CheeksTheGeek]

give first-class integration of copra to cocotb testbenches so users get stubs by simply running make

[cmarqu]

@cocotb/maintainers Do we still want to improve the Makefiles with new features, or are they in maintenance mode?

[ktbarrett]

I'd say judicious addition of new features. I think it's fine here. We are still using the Makefiles in our regression so I'd like to see new features there until we finally replace them all.