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

The above is very helpful. I've spent a bit of time with the branch, a couple comments inline. I think we can remove the source sanitize in install.py (it's done in extract). And then I started on expanding the documentation.

It was very interesting to see how the implementation works; and I want to document the expectations between "main-layers" and other images. There are some interesting side-effects that I think we must document. In particular, only main images seem to have peers at the same level, for example showing nested main layers; that is we cannot put a set of main layers in a sublayer. Pointing at any of the sub-images in the tree below only gets the sub-image, no peers nor parents, is that expected?

├── 10-first
│   ├── 10-main-nest1.img
│   ├── 20-main-nest2
│   │   └── 10-main-nest2-nest1.img
│   ├── 20-main-nest2.img
│   └── 30-main-test3.img
├── 10-first.img
├── 20-second
│   ├── 20-second-nest1
│   │   ├── 30-second-nest1-nest2
│   │   │   └── 40-second-nest1-nest2-nest3.img
│   │   └── 30-second-nest1-nest2.img
│   └── 20-second-nest1.img
└── 20-second.img