kbuild: do not create orphan built-in.a or obj-y objects

Both 'obj-y += foo/' and 'obj-m += foo/' request Kbuild to visit the
sub-directory foo/, but the difference is that only the former combines
foo/built-in.a into the built-in.a of the current directory because
everything in sub-directories visited by obj-m is supposed to be modular.

So, it makes sense to create built-in.a only if that sub-directory is
reachable by the chain of obj-y. Otherwise, built-in.a will not be
linked into vmlinux anyway. For the same reason, it is pointless to
compile obj-y objects in the directory visited by obj-m.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.build

@@ -66,7 +66,7 @@ lib-target := $(obj)/lib.a
 real-obj-y += $(obj)/lib-ksyms.o
 endif
 
-ifneq ($(strip $(real-obj-y) $(need-builtin)),)
+ifdef need-builtin
 builtin-target := $(obj)/built-in.a
 endif