Bug#759406: Image sizes and concatenation

[Josh Triplett <josh@joshtriplett.org>]

I was the person at that DebConf session who suggested concatenatable
images.  And in particular, if you want to handle media of power-of-two
sizes (1GB, 2GB, 4GB, 8GB, 16GB), you can provide concatenatable images
of power-of-two sizes after the first: 1GB, 1GB, 2GB, 4GB, 8GB.  More
generally, if you want to support sizes a, b, c, d, e, ..., then provide
images of size a, b-a, c-b, d-c, e-d, ...

Offsets were exactly what I had in mind when I suggested that this
seemed easily fixable: you can tell how big the first image is supposed
to be, so just look right after that image for another image, and repeat
until you don't find another image.

Thanks for working on this!

- Josh Triplett