[Toaster] Design - Downloading custom image recipe files

[Barros Pena, Belen]

If we don't want custom images to be stuck in Toaster, we need to allow
people to download their recipe files. I've opened this enhancement for
that

https://bugzilla.yoctoproject.org/show_bug.cgi?id=8128

The design document is attached to the Bugzilla entry as usual. Any
questions / comments, let me know.

Thanks!

Belén