kbuild: fix directory traversal bug

Previously kbuild choked over the following:
obj-y += ../../../arch/i386/kernel/bootflag.o

This has resulted in some rather ugly workarounds in
current x86_64 tree.
This patch fixes kbuild to allow the above and enable
potential cleanups in x86_64 and maybe in other places.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/Makefile.lib

@@ -44,9 +44,9 @@ multi-objs-y := $(foreach m, $(multi-used-y), $($(m:.o=-objs)) $($(m:.o=-y)))
 multi-objs-m := $(foreach m, $(multi-used-m), $($(m:.o=-objs)) $($(m:.o=-y)))
 multi-objs   := $(multi-objs-y) $(multi-objs-m)
 
-# $(subdir-obj-y) is the list of objects in $(obj-y) which do not live
-# in the local directory
-subdir-obj-y := $(foreach o,$(obj-y),$(if $(filter-out $(o),$(notdir $(o))),$(o)))
+# $(subdir-obj-y) is the list of objects in $(obj-y) which uses dir/ to
+# tell kbuild to descend
+subdir-obj-y := $(filter %/built-in.o, $(obj-y))
 
 # $(obj-dirs) is a list of directories that contain object files
 obj-dirs := $(dir $(multi-objs) $(subdir-obj-y))