kbuild: make multiple directory targets work

Currently, the single-target build does not work when two
or more sub-directories are given:

  $ make fs/ kernel/ lib/
    CALL    scripts/checksyscalls.sh
    CALL    scripts/atomic/check-atomics.sh
    DESCEND  objtool
  make[2]: Nothing to be done for 'kernel/'.
  make[2]: Nothing to be done for 'fs/'.
  make[2]: Nothing to be done for 'lib/'.

Make it work properly.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1679,7 +1679,7 @@ PHONY += descend $(build-dirs)
 descend: $(build-dirs)
 $(build-dirs): prepare
 	$(Q)$(MAKE) $(build)=$@ \
-	single-build=$(if $(filter-out $@/, $(single-no-ko)),1) \
+	single-build=$(if $(filter-out $@/, $(filter $@/%, $(single-no-ko))),1) \
 	need-builtin=1 need-modorder=1
 
 clean-dirs := $(addprefix _clean_, $(clean-dirs))