kbuild: allow to run dt_binding_check without kernel configuration

The dt_binding_check target is located outside of the
'ifneq ($(dtstree),) ... endif' block.

So, you can run 'make dt_binding_check' on any architecture.
This makes a perfect sense because the dt-schema is arch-agnostic.

The only one problem I see is that scripts/dtc/dtc is not always built.
For example, ARCH=x86 defconfig does not define CONFIG_DTC. Kbuild
descends into scripts/dtc/ with doing nothing. Then, it fails to build
*.example.dt.yaml files.

Let's build scripts/dtc/dtc forcibly when running dt_binding_check.

The dt-schema does not depend on any CONFIG option either, so you
should be able to run dt_binding_check without the .config file.

Going forward, you can directly run 'make dt_binding_check' in a
pristine source tree.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>

Makefile

@@ -255,7 +255,7 @@ clean-targets := %clean mrproper cleandocs
 no-dot-config-targets := $(clean-targets) \
 			 cscope gtags TAGS tags help% %docs check% coccicheck \
 			 $(version_h) headers headers_% archheaders archscripts \
-			 %asm-generic kernelversion %src-pkg
+			 %asm-generic kernelversion %src-pkg dt_binding_check
 no-sync-config-targets := $(no-dot-config-targets) install %install \
 			   kernelrelease
 single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.s %.symtypes %/

scripts/dtc/Makefile

 # SPDX-License-Identifier: GPL-2.0
 # scripts/dtc makefile
 
-hostprogs		:= dtc
-always-$(CONFIG_DTC)	:= $(hostprogs)
+hostprogs			:= dtc
+always-$(CONFIG_DTC)		+= $(hostprogs)
+always-$(CHECK_DT_BINDING)	+= $(hostprogs)
 
 dtc-objs	:= dtc.o flattree.o fstree.o data.o livetree.o treesource.o \
 		   srcpos.o checks.o util.o