RFR: JDK-8010185 add --with-override-nashorn to configure

[Magnus Ihse Bursie]

When nashorn was added, we should extend the --with-override-X 
functionality to cover it as well. An attempt to do so was appearantly 
made, but it did not work.

This fix also adds checks for nashorn analogue to the other repos for 
--with-add/override-source-root.

Webrev here:

http://cr.openjdk.java.net/~ihse/JDK-8010185-add-with-override-nashorn-to-configure/webrev.01

/Magnus