Introduce the DIB_ namespace for build-time config.

Document the use of DIB_ as a prefix on environmental variables that can
be overridden at build time.

Introduce the first such variable by migrating IMAGE_SIZE to
DIB_IMAGE_SIZE.

Change-Id: Ie36b734991b913a23f37f2add47d470d7c1576e4
Merge "Introduce the DIB_ namespace for build-time config."