kernel-build.mk: there is no tarball when using CONFIG_KERNEL_GIT_CLONE_URI or CONFIG_EXTERNAL_KERNEL_TREE, do not make the prepared stamp file depend on it in this case

SVN-Revision: 28361

include/kernel-build.mk

@@ -61,7 +61,7 @@ define BuildKernel
   $(if $(QUILT),$(Build/Quilt))
   $(if $(LINUX_SITE),$(call Download,kernel))
 
-  $(STAMP_PREPARED): $(DL_DIR)/$(LINUX_SOURCE)
+  $(STAMP_PREPARED): $(if $(LINUX_SITE),$(DL_DIR)/$(LINUX_SOURCE))
 	-rm -rf $(KERNEL_BUILD_DIR)
 	-mkdir -p $(KERNEL_BUILD_DIR)
 	$(Kernel/Prepare)