Add support for build= variable

build is a magic variable that acts similarly to subdirs, but specifies
files to build instead of subdirs.  Using files provides a few advantages:
files can be named something more appropriate for the project than
"Blueprints", whatever you want instead of Blueprints, and multiple
Blueprints files can exist in the same directory.

A new variable is used instead of putting filenames into subdirs to avoid
unexpected behavior when a glob matches both files and directories.

subdirs= and build= entries that don't match any directories or files
are now reported as errors.

Change-Id: Id329504ace251eab4ccea1081a3c8665a4c52f5a