RFR: JDK-8149963: build errors during API docs build

[Tim Bell]

Erik:

> There is a dependency issue in the Javadoc makefile when building 
> custom additions. Since the include of the custom Javadoc.gmk happens 
> before the core api build rule declaration, the targets in the custom 
> file cannot properly depend on the core api. The fix is to move the 
> variable definition of the core api javadoc target to before the include.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8149963
> Webrev: http://cr.openjdk.java.net/~erikj/8149963/webrev.01/

Looks good to me.

Tim