[PATCH 1 of 2] config: introduce load order tracking

[Yuya Nishihara]

On Tue, 10 Jul 2018 07:58:04 -0700, Martin von Zweigbergk wrote:
> I'd also be happy to see the immutable configs series land. Yuya, do you
> remember what about it you found over engineered? Perhaps it wouldn't be
> hard to fix those things and land it.

It isn't landed just because Jun got busy for the other work. I don't
have any particular concern, but I just felt it could be slightly simpler.