Add opt-in option to build for one target only

Almost all of the hard work has been done in #534 and #532. This would allow merging a non-breaking version of #532, enabling cool things like #563 and possibly other issues tagged with `uses-more-storage` (#616, #608 come to mind).

The only thing that needs to change is updating the documentation and still using `TARGETS` instead of only `default-target` if `extra-targets` is unset.