[Toaster] Design - Deleting custom images

[Barros Pena, Belen]

I've finished documenting the last bit of the image customisation feature:
deleting custom images. I've opened this Bugzilla enhancement:

https://bugzilla.yoctoproject.org/show_bug.cgi?id=8132

The design document is attached to it, as usual. What I am proposing is
kind of non-standard, so if you have a chance, please have a look at it.
The document is quite short.

Questions / comments, let me know.

Thanks!

Belén