RFR: JDK-8219129: Allow overriding of license files in legal dir

[Erik Joelsson]

Like with sources in java and native compilation, this patch allows 
automatic override of license files in the legal dir by just providing a 
"custom" file with the same name.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219129
Webrev: http://cr.openjdk.java.net/~erikj/8219129/webrev.01/

/Erik