OpenJDK release file has empty SOURCE field

When building promoted openjdk builds in jdk 10 through mach5, the SOURCE field of the release file ends up empty. This is because of the new source layout in the consolidated forest. A related issue was also present in 9u, but the consolidated forest makes it behave quite differently in 10. See JDK-8191940.

In JDK 10, the .src-rev file now ends up in the closed root, which causes the openjdk source bundle to not include any .src-rev.