[oe] A new bitbake extension: .bbappend files

[Frans Meulenbroeks]

Wrt the bbappend / amend.inc discussion:

I was wondering how version management would work.
I can imagine that if I change something in a bbappend file (or amend.inc)
file, I would like to have something like PR that I could bump.
In my case these recipes live in an overlay which is also used by
co-workers. I would like to be able to trigger a rebuild when they get a
modified .bbappend file.

Frans