failed builds should be automatically removed, unless otherwise specified

[micahsnyder]

If a build successfully passes the configure step but fails the make or install steps, then it should be removed from the work directly, unless the user wants it to stick around so they can investigate why it failed in more depth. The reason being that if you're working on a recipe, a subsequent build will continue to fail unless you do -r. If your testing with a much larger build (like a collection), that -r will apply to the whoooole thing, resulting in a full rebuild of everything which takes forever. It would be better for the failed build to disappear automatically so a new build attempt will start fresh with the configure step again.

I would propose that we add a flag like msl build <recipe> --keep-failed-builds for those who want to keep the broken build around to investigate it.