kbuild: Support flat DTBs install

In preparation to move Arm .dts files into sub-directories grouped
by vendor/family, the current flat tree of DTBs generated by
dtbs_install needs to be maintained. Moving the installed DTBs to
sub-directories would break various consumers using 'make dtbs_install'.

This is a NOP until sub-directories are introduced.
Signed-off-by: Rob Herring <robh@kernel.org>

arch/arm/Kconfig

@@ -1460,6 +1460,9 @@ config USE_OF
 	help
 	  Include support for flattened device tree machine descriptions.
 
+config ARCH_WANT_FLAT_DTB_INSTALL
+	def_bool y
+
 config ATAGS
 	bool "Support for the traditional ATAGS boot data passing"
 	default y

scripts/Makefile.dtbinst

@@ -34,6 +34,6 @@ $(dst)/%.dtbo: $(obj)/%.dtbo
 
 PHONY += $(subdirs)
 $(subdirs):
-	$(Q)$(MAKE) $(dtbinst)=$@ dst=$(patsubst $(obj)/%,$(dst)/%,$@)
+	$(Q)$(MAKE) $(dtbinst)=$@ dst=$(if $(CONFIG_ARCH_WANT_FLAT_DTB_INSTALL),$(dst),$(patsubst $(obj)/%,$(dst)/%,$@))
 
 .PHONY: $(PHONY)