kbuild: fix single target builds for external modules

Commit f566e1f ("kbuild: make multiple directory targets work")
broke single target builds for external modules. Fix this.

Fixes: f566e1f ("kbuild: make multiple directory targets work")
Reported-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Bjørn Mork <bjorn@mork.no>

Makefile

@@ -1754,7 +1754,7 @@ PHONY += descend $(build-dirs)
 descend: $(build-dirs)
 $(build-dirs): prepare
 	$(Q)$(MAKE) $(build)=$@ \
-	single-build=$(if $(filter-out $@/, $(filter $@/%, $(single-no-ko))),1) \
+	single-build=$(if $(filter-out $@/, $(filter $@/%, $(KBUILD_SINGLE_TARGETS))),1) \
 	need-builtin=1 need-modorder=1
 
 clean-dirs := $(addprefix _clean_, $(clean-dirs))