RFR: JDK-8073231 Add convenient way of adding custom configure options to jprt

[Ingemar Aberg]

A convenient way of adding configure options when building JDK with jprt 
is needed, for example to build colocated tests

Bug: https://bugs.openjdk.java.net/browse/JDK-8073231
Webrev: http://cr.openjdk.java.net/~erikj/8073231/webrev.00/