kbuild: handle dtb-y and CONFIG_OF_ALL_DTBS natively in Makefile.lib

If CONFIG_OF_ALL_DTBS is enabled, "make ARCH=arm64 dtbs" compiles each
DTB twice; one from arch/arm64/boot/dts/*/Makefile and the other from
the dtb-$(CONFIG_OF_ALL_DTBS) line in arch/arm64/boot/dts/Makefile.
It could be a race problem when building DTBS in parallel.

Another minor issue is CONFIG_OF_ALL_DTBS covers only *.dts in vendor
sub-directories, so this broke when Broadcom added one more hierarchy
in arch/arm64/boot/dts/broadcom/<soc>/.

One idea to fix the issues in a clean way is to move DTB handling
to Kbuild core scripts.  Makefile.dtbinst already recognizes dtb-y
natively, so it should not hurt to do so.

Add $(dtb-y) to extra-y, and $(dtb-) as well if CONFIG_OF_ALL_DTBS is
enabled.  All clutter things in Makefiles go away.

As a bonus clean-up, I also removed dts-dirs.  Just use subdir-y
directly to traverse sub-directories.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
[robh: corrected BUILTIN_DTB to CONFIG_BUILTIN_DTB]
Signed-off-by: Rob Herring <robh@kernel.org>

arch/h8300/boot/dts/Makefile

@@ -7,8 +7,3 @@ obj-y += $(BUILTIN_DTB)
 dtb-$(CONFIG_H8300H_SIM) := h8300h_sim.dtb
 dtb-$(CONFIG_H8S_SIM) := h8s_sim.dtb
 dtb-$(CONFIG_H8S_EDOSK2674) := edosk2674.dtb
-
-dtstree		:= $(srctree)/$(src)
-dtb-$(CONFIG_OF_ALL_DTBS) := $(patsubst $(dtstree)/%.dts,%.dtb, $(wildcard $(dtstree)/*.dts))
-
-always	    := $(dtb-y)