kbuild: move subdir-obj-y to scripts/Makefile.build

Save $(addprefix ...) for subdir-obj-y.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index f46d254..ee283ef 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -69,6 +69,8 @@
 
 # ===========================================================================
 
+subdir-obj-y := $(filter %/built-in.a, $(real-obj-y))
+
 ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)
 lib-target := $(obj)/lib.a
 endif