Fix incremental build number on Darwin

Darwin does not include sha1sum, but it should have openssl.

Change-Id: If2b685c1478f9486bed21dfba0c655d9e02db72b
Merge "Fix incremental build number on Darwin" into cm-10.2
Use openssl instead of sha1sum

Change-Id: Iab8a74d4af6646104cb23312853f811a38bdec0d