JDK-8199539 broke the OpenJDK build

The fix in JDK-8199539 works fine for OracleJDK builds but broke the OpenJDK build.

The problem is that I used CUSTOM_ROOT as the SRC dir for SetupCopyFiles. This was a bit of a hack together with FLATTEN := true. The idea was that I didn't care about the source dir but just wanted each file listed copied into the dest dir. CUSTOM_ROOT is however only defined in the OracleJDK build so won't work in an open only build.