RFR: 8226325: Support building of filtered spec bundles

[Dan Smith]

See this webrev for some enhancements to the documentation-building 
capabilities of the build system.

http://cr.openjdk.java.net/~dlsmith/jdk/8226325/webrev.00 
<http://cr.openjdk.java.net/~dlsmith/jdk/8226325/webrev.00>