kbuild: consolidate Devicetree dtb build rules

There is nothing arch specific about building dtb files other than their
location under /arch/*/boot/dts/. Keeping each arch aligned is a pain.
The dependencies and supported targets are all slightly different.
Also, a cross-compiler for each arch is needed, but really the host
compiler preprocessor is perfectly fine for building dtbs. Move the
build rules to a common location and remove the arch specific ones. This
is done in a single step to avoid warnings about overriding rules.

The build dependencies had been a mixture of 'scripts' and/or 'prepare'.
These pull in several dependencies some of which need a target compiler
(specifically devicetable-offsets.h) and aren't needed to build dtbs.
All that is really needed is dtc, so adjust the dependencies to only be
dtc.

This change enables support 'dtbs_install' on some arches which were
missing the target.

Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Paul Burton <paul.burton@mips.com>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: linux-kbuild@vger.kernel.org
Cc: linux-snps-arc@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: uclinux-h8-devel@lists.sourceforge.jp
Cc: linux-mips@linux-mips.org
Cc: nios2-dev@lists.rocketboards.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-xtensa@linux-xtensa.org
Signed-off-by: Rob Herring <robh@kernel.org>

arch/nios2/Makefile

@@ -56,12 +56,6 @@ all: vmImage
 archclean:
 	$(Q)$(MAKE) $(clean)=$(nios2-boot)
 
-%.dtb %.dtb.S %.dtb.o: | scripts
-	$(Q)$(MAKE) $(build)=$(nios2-boot)/dts $(nios2-boot)/dts/$@
-
-dtbs:
-	$(Q)$(MAKE) $(build)=$(nios2-boot)/dts
-
 $(BOOT_TARGETS): vmlinux
 	$(Q)$(MAKE) $(build)=$(nios2-boot) $(nios2-boot)/$@
 
@@ -74,5 +68,4 @@ define archhelp
   echo  '                     (your) ~/bin/$(INSTALLKERNEL) or'
   echo  '                     (distribution) /sbin/$(INSTALLKERNEL) or'
   echo  '                     install to $$(INSTALL_PATH)'
-  echo  '  dtbs            - Build device tree blobs for enabled boards'
 endef

arch/nios2/boot/Makefile

@@ -31,9 +31,5 @@ $(obj)/zImage: $(obj)/compressed/vmlinux FORCE
 $(obj)/compressed/vmlinux: $(obj)/vmlinux.gz FORCE
 	$(Q)$(MAKE) $(build)=$(obj)/compressed $@
 
-targets += $(dtb-y)
-
-$(obj)/dtbs: $(addprefix $(obj)/, $(dtb-y))
-
 install:
 	sh $(srctree)/$(src)/install.sh $(KERNELRELEASE) $(BOOTIMAGE) System.map "$(INSTALL_PATH)"