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 4b024242e8 ("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 <[email protected]>
Reviewed-by: Sam Ravnborg <[email protected]>

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))"