ch-build2dir usage is terrible

It's clunky, inflexible, and inconsistent with the other scripts. Why isn't it just like ch-build but with an output directory specified?

I think this has contaminated the documentation as well. There are places with a ch-build, ch-docker2tar, ch-tar2dir sequence that's not supported by ch-build2dir currently but should be. Changes to the latter should fix the docs too.