[OE-core] RFC: Layer tooling brainstorming

[Jeremy Puhlman]

So we(montavista) basically have run through this process before, though
using collections implementation. Difference in overall implementation
aside, the issues are largely the same.

> * This layer tooling probably belongs at a higher level on the stack 
>   than bitbake itself

This is most definately true, and has worked well for us. We have a set
of content tools that describes the distribution of each of the
collections and prebuilt componets that get shipped to the end user.

> * Maybe need to split into "bootstrap" steps (e.g where pseduo is 
>   established, layers downloaded etc)

Internally we extended collections to act upon remote collections of
with any kind fetching bitbake supports. The collections implementation
has a "built-in" bootstrap step. The actual nice thing about the layers
implimentation, is you could implement the remote fetching with out
having the re-execution of bitbake. That being said there is a clear
need for a method of adding bootstrap steps to address things like
pseudo. I think trying to put something extensible here would probably
be good.