RFR: JDK-8041487

[Erik Joelsson]

I recently had to work with make/Javadoc.gmk and felt that it needed 
some attention. This patch makes it behave correctly for incremental 
builds and reduces the log output on the default log level to much more 
manageable levels.

I realize that we should probably rewrite this even more to reduce all 
the code duplication, but choose to leave that for later.

Bug: https://bugs.openjdk.java.net/browse/JDK-8041487
Webrev: http://cr.openjdk.java.net/~erikj/8041487/webrev.root.01/

/Erik