Sandbox makefile: Doc update.
Sandbox makefile: Added notes about build phases.
Sandbox makefile updates.

- Removed downloaded archive sentry. This was originally because
clang had 3 archives to download. The current setup doesn't need
sentries because make will delete a partially-downloaded file if
the result is nonzero or the download is interrupted.
Sandbox makefile: Bashed lib functions, moved to versions.mk.
Sandbox makefile: Bashed lib.