introduce a new build mechanism

This patch replaces the collection of individual "build" scripts with a
single script (tools/build-docker-image), made available as "build"
inside each image directory.

The build-docker-image script will, by default, build images tagged with
the current commit id in order to prevent developers from accidentally
stepping on each other or on release images.

Documentation in docs/image-building.md describes the script in more
detail.

Change-Id: I444d5c2256a85223f8750a0904cb4b07f18ab67f