A mistake in JDK-8166948 is causing jmods and images to always be rebuilt on subsequent calls to make when nothing should have changed. The fix is small and simple:

diff -r b913840943c0 make/ExplodedImageOptimize.gmk
--- a/make/ExplodedImageOptimize.gmk
+++ b/make/ExplodedImageOptimize.gmk
@@ -39,6 +39,7 @@
 $(PACKAGES_ATTRIBUTE_TARGET): $(ALL_MODULEINFO_CLASSES) $(BUILD_JIGSAW_CLASSES)
  $(call LogInfo, Optimizing the exploded image)
  $(TOOL_ADD_PACKAGES_ATTRIBUTE) $(JDK_OUTPUTDIR)
+ $(TOUCH) $@
 
 TARGETS := $(PACKAGES_ATTRIBUTE_TARGET)