[Toaster] Design - Creating custom images

[Barros Pena, Belen]

I've documented the process of creating a custom image with Toaster, and
opened a Bugzilla enhancement for it

https://bugzilla.yoctoproject.org/show_bug.cgi?id=8103

I am not sure if this requires any additional back end work. If it does,
please say so: I can open any extra enhancements needed.

The design document is attached to the Bugzilla entry, as usual. Please
have a look at it if you can, since it involves changes to quite a few
Toaster pages. The document also explains the meta-custom-images layer (on
page 17).

If you have any questions / comments, let me know.

Thanks!

Belén