refactor(build_image): Build dev and prod images independently

We need some more control over exactly what lands in dev vs prod images
which will require letting them diverge in what is currently the common
base image step. There isn't any real need for the base image in the
first place other than to speed up building both dev and prod images at
the same time but that isn't common enough to worry about.

As part of this cleanup also remove references to CHROMEOS_* variables
and the recovery image that never actually existed in CoreOS.
refactor(base_image_util): Split into two functions.

Use what was the base image build function as setup/finalize steps in
the dev and prod build functions. This eliminates duplicate code
that mounted and unmounted the filesystem images.
refactor(build_image_util): Merge with base_image_util

There isn't a base image now, and less file clutter is nice.
cleanup(build_image_util): Delete unused code
refactor(build_image_util): Move prod specific code to prod_image_util
Merge pull request #250 from marineam/refactor

Refactor image building