RFR: JDK-8176849: jdk9 BCL builds fail after cleaning up temporary file ASSEMBLY_EXCEPTION

[Erik Joelsson]

When building OpenJDK source after converting it to a different license, 
the build fails because the file ASSEMBLY_EXCEPTION is missing. This 
change makes the build skip including top level license files if they 
don't exist.

Bug: https://bugs.openjdk.java.net/browse/JDK-8176849

Patch:

diff -r 5548e024cbcf make/copy/Copy-java.base.gmk
--- a/make/copy/Copy-java.base.gmk
+++ b/make/copy/Copy-java.base.gmk
@@ -235,8 +235,10 @@

  # JDK license and assembly exception files to be packaged in JMOD

-JDK_LICENSE ?= $(TOPDIR)/LICENSE
-JDK_NOTICE  ?= $(TOPDIR)/ASSEMBLY_EXCEPTION
+# The license files may not be present if the source has been obtained 
using a
+# different license.
+JDK_LICENSE ?= $(wildcard $(TOPDIR)/LICENSE)
+JDK_NOTICE  ?= $(wildcard $(TOPDIR)/ASSEMBLY_EXCEPTION)

  $(eval $(call SetupCopyFiles, COPY_JDK_NOTICES, \
      FILES := $(JDK_LICENSE) $(JDK_NOTICE), \