Partial rebuilds

[anuejn]

It would be nice, if the gateware wouldnt be rebuild if only pydriver changed.
It is currently unclear to me how to do that in a clean way

[rroohhh]

I think this could potentially be solved by by using amigen.

amigen more cleanly seperates extra information from the actual gateware.

What amigen does not currently really have is a structured way to generate extra parts belonging to the gateware, but not part of the actual gateware (like the pydriver files). I think we should think of how we want to API there to look like. Currently amigen is used by simply converting everything into a nmigen Module in the end. Maybe we want to additionally output some custom information generated by the various amigen Elements? Or maybe we want to add our own custom build plan system?