RFR: JDK-8192771: Boot JDK jar tool used to construct the modular JAR for java.jnlp

[Erik Joelsson]

To be able to bump the classfile version in JDK 10, we need to be able 
to build certain jar files using the newly built jar tool during the 
build. This patch enables overriding of the jar cmd in the build.

Bug: https://bugs.openjdk.java.net/browse/JDK-8192771

Webrev: http://cr.openjdk.java.net/~erikj/8192771/webrev.01/

/Erik