RFR: JDK-8231974: Build fails if no common legal notices are present

[Erik Joelsson]

In certain Oracle build configurations, the common legal output 
directory is never created, because there are no legal files to put in 
it. This should not make the build fail. This patch makes adding the 
--legal-notices parameter to jmod optional.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231974

Webrev: http://cr.openjdk.java.net/~erikj/8231974/webrev.01/

/Erik