Allow overwriting old images

The two duplicated functions, save_image and finish_image will move
an existing image out of the way if it exists, but it isn't
configurable. Check an environment variable is 0 before doing so.

Switch save_image to just calling finish_image, rather than
duplicating its code exactly.

Change-Id: I26a5a8fa4b6e853c9440bffab195b0bc3728be40
Detail 'other' directories in the README

Elements can use arbitrary directories in diskimage-builder, so
ensure we make mention of that, and give an example of such a use.

Change-Id: I0e57ece82629c1be4e0f9612c2a565802b222687
Merge "Allow overwriting old images"
Merge "Detail 'other' directories in the README"