kbuild: create built-in.o automatically if parent directory wants it

"obj-y += foo/" syntax requires Kbuild to visit the "foo" subdirectory
and link built-in.o from that directory.  This means foo/Makefile is
responsible for creating built-in.o even if there is no object to
link (in this case, built-in.o is an empty archive).

We have had several fixups like commit 4b024242 ("kbuild: Fix
linking error built-in.o no such file or directory"), then ended up
with a complex condition as follows:

  ifneq ($(strip $(obj-y) $(obj-m) $(obj-) $(subdir-m) $(lib-target)),)
  builtin-target := $(obj)/built-in.o
  endif

We still have more cases not covered by the above, so we need to add
  obj- := dummy.o
in several places just for creating empty built-in.o.

A key point is, the parent Makefile knows whether built-in.o is needed
or not.  If a subdirectory needs to create built-in.o, its parent can
tell the fact when descending.

If non-empty $(need-builtin) flag is passed from the parent, built-in.o
should be created.  $(obj-y) should be still checked to support the
single target "%/".  All of ugly tricks will go away.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1009,7 +1009,7 @@ $(sort $(vmlinux-deps)): $(vmlinux-dirs) ;
 
 PHONY += $(vmlinux-dirs)
 $(vmlinux-dirs): prepare scripts
-	$(Q)$(MAKE) $(build)=$@
+	$(Q)$(MAKE) $(build)=$@ need-builtin=1
 
 define filechk_kernel.release
 	echo "$(KERNELVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"

scripts/Makefile.build

@@ -76,7 +76,7 @@ lib-target := $(obj)/lib.a
 obj-y += $(obj)/lib-ksyms.o
 endif
 
-ifneq ($(strip $(obj-y) $(obj-m) $(obj-) $(subdir-m) $(lib-target)),)
+ifneq ($(strip $(obj-y) $(need-builtin)),)
 builtin-target := $(obj)/built-in.o
 endif
 
@@ -561,7 +561,7 @@ targets := $(filter-out $(PHONY), $(targets))
 
 PHONY += $(subdir-ym)
 $(subdir-ym):
-	$(Q)$(MAKE) $(build)=$@
+	$(Q)$(MAKE) $(build)=$@ need-builtin=$(if $(findstring $@,$(subdir-obj-y)),1)
 
 # Add FORCE to the prequisites of a target to force it to be always rebuilt.
 # ---------------------------------------------------------------------------