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

[Erik Joelsson]

Hello,

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/

/Erik