kbuild: clear KBUILD_MODULES in top Makefile if CONFIG_MODULES=n

Do not try to build any module-related artifacts when CONFIG_MODULES
is disabled.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 9af88f4..f46d254 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -77,7 +77,7 @@
 builtin-target := $(obj)/built-in.a
 endif
 
-ifeq ($(CONFIG_MODULES)$(need-modorder),y1)
+ifdef need-modorder
 modorder-target := $(obj)/modules.order
 endif