Code review comment for ~jibel/curtin/+git/add-layer-images-support:master

This looks quite straight forward. Thanks.

I would like to see the parts of the main extract_root_* broken up into
seperate functions so we can add unittests for each of those portions.

Specifically:

I'd like to unittest the downloading of images (do we call download on the items provided by the path stack), what happens if one of the downloads fail, what happens when download works but file is empty.

I'd also like a unittest over the main extract_root_*;