Add a main wrapper function to return 1 if any images failed to build;
otherwise, return 0. Rename the old main() to run_build(), which still
returns image statuses, to avoid interfering with unit tests.

Change-Id: I5817cc9cebb4f42dc13e0535223e5dc8931a3aab
Co-Authored-By: Mark Goddard <mgoddard@cray.com>
Closes-Bug: #1578404