Add the makefile bits in functoria

[Drup]

Currently, it's in mirage. It needs to be careful about custom configuration.

[hannesm]

@Drup still planned? I'd say we can close this now, the opam generation is part of functoria.

[Drup]

opam give you packaging/dependency management, not building.

This is a feature wish, there is no reason to close it, even if nobody is working on it.

[hannesm]

but building is now handled throughout build in the device graph. I don't see what generating a Makefile in functoria would give you (but hard to see without a picture in my head how a non-mirage use case would look like)