RFR: JDK-8199539: Provide a standard way for the build to filter un-needed legal .md files

[Erik Joelsson]

When bundling legal files, we need to filter them according to what 3rd 
party components are actually included in the build. Several components, 
such as zlib, libjpeg and freetype can be linked with system libraries 
instead, and if so, we should also exclude the legal files for that 
component.

This patch introduces a optional copy/filter step in the <module>-copy 
targets. Since there are only two affected modules at this time I didn't 
bother unifying this into one implementation. Instead the short logic is 
duplicated between the two.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199539

Webrev: http://cr.openjdk.java.net/~erikj/8199539/webrev.01/index.html

Bug: https://bugs.openjdk.java.net/browse/JDK-8199539

/Erik