Review Request: 8004045: build-infra: Error 12 from zip when updating src.zip

[Erik Joelsson]

Small fix for common annoyance when rebuilding images. The zip utility 
returns 12 when it has nothing to do. This happens because we run it 
multiple times, once for each source root, when updating src.zip, even 
if only sources in one of the roots have changed.

http://cr.openjdk.java.net/~erikj/8004045/webrev.root.01/
/Erik