kbuild: fix make -jN with multiple targets with O=...

The way multiple targets was handled with make O=...
broke because for each high-level target make spawned
a parallel make resulting in a broken build.
Reported by Keith Owens <kaos@ocs.com.au>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -106,12 +106,13 @@ KBUILD_OUTPUT := $(shell cd $(KBUILD_OUTPUT) && /bin/pwd)
 $(if $(KBUILD_OUTPUT),, \
      $(error output directory "$(saved-output)" does not exist))
 
-.PHONY: $(MAKECMDGOALS)
+.PHONY: $(MAKECMDGOALS) cdbuilddir
+$(MAKECMDGOALS) _all: cdbuilddir
 
-$(filter-out _all,$(MAKECMDGOALS)) _all:
+cdbuilddir:
 	$(if $(KBUILD_VERBOSE:1=),@)$(MAKE) -C $(KBUILD_OUTPUT) \
 	KBUILD_SRC=$(CURDIR) \
-	KBUILD_EXTMOD="$(KBUILD_EXTMOD)" -f $(CURDIR)/Makefile $@
+	KBUILD_EXTMOD="$(KBUILD_EXTMOD)" -f $(CURDIR)/Makefile $(MAKECMDGOALS)
 
 # Leave processing to above invocation of make
 skip-makefile := 1