Optionally override copyright header in generated source

In the JDK build we have various build tools that generate source code from data files. For most of these tools, the source files are based on template files, which already have copyright headers, but for some, the complete source file is generated by the tool, which is providing the copyright header programatically. For the latter, we would like to implement an override mechanism in each tool so that we can change the copyright header from a custom makefile.