matthew-hardware's common/generic-uefi-image.nix still sets
image.repart.enable = true; nixpkgs removed that option (importing
image/repart.nix now always defines system.build.image). Declare it so
the definition has somewhere to land. Drop once upstream stops setting it.
Cross build verified end to end (toplevel + system.build.image).
kurogeek
fd3a7034e8