The only files in the docker image directories that differ majorly between
distros are the Dockerfiles (e.g. different package manager mechanisms,
different repo sources, etc).

The supporting files such as the start.sh/check.sh and config files should
largely stay the same between base images.

This change moves these files up into a "common" directory, and symlinks them
in the same way that the build script is managed. This means when adding a new
base distro only new Dockerfiles need to be created. Also, if a distro does
happen to require a vastly different start.sh script for example, one can
choose to simply not symlink to the common and instead drop in a custom one.

Implements: blueprint refactor-base-image-layout

Change-Id: Ic4db69d31ff54a1fb95af4853a2e5ae490064284