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

Adding a new source type was our initial thought too. But it finally made more sense to extend the existing fsimage.
Ultimately the API is the same and a path to a single filesystem image is passed as an installation source. The user doesn't have to have the knowledge about the type of build (flat image or layered). Moreover the code to handle a flat image or a layered image in Curtin is very similar, the main difference being the computation of the path then merging with an overlay. It reduces the redundancy of the code and its overall complexity is factorized into this single fsimage type.