NAS: Add lustre version to the BUILD_VERSION

I have been using DOWNSTREAMS_RELEASE as BUILD_VERSION. It provides
true build version within a lustre release.

Some community tool that NASA Ames uses on Lustre expects to see
<lustre_version>-<build_version> in BUILD_VERSION string. This patch
makes those tools happy.

Signed-off-by: Jay Lan <jay.j.lan@nasa.gov>