[Toaster] Design - Building custom images

[Barros Pena, Belen]

I've documented the process of building a custom image and opened

https://bugzilla.yoctoproject.org/show_bug.cgi?id=8104

The design document is attached to the Bugzilla entry (as usual). Nothing
controversial in it, I hope, although you might want to look at pages 7 to
9, which talk about the meta-custom-images layer.

Any questions or comments, let me know.

Thanks,

Belén