kbuild: Optionally enable schema checks for %.dtb targets

While not documented, schema checks for single dtb targets mostly work
already by setting 'CHECK_DTBS=1'. However, the dependencies are not
handled and it only works if 'make dt_bindings_check' was run first and
generated processed-schema.json. In addition, changing a binding file
doesn't cause the schema to be rebuilt and dtb to be revalidated.

Making this work turns out to be simple. Whenever CHECK_DTBS is set,
make 'dt_binding_check' a 'dtbs_prepare' dependency.

I reimplemented here what Masahiro had originally come up with a while
back.

Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20221220013233.2890335-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>

Makefile

@@ -1481,7 +1481,10 @@ dtbs_prepare: include/config/kernel.release scripts_dtc
 
 ifneq ($(filter dtbs_check, $(MAKECMDGOALS)),)
 export CHECK_DTBS=y
-dtbs: dt_binding_check
+endif
+
+ifneq ($(CHECK_DTBS),)
+dtbs_prepare: dt_binding_check
 endif
 
 dtbs_check: dtbs
@@ -1779,6 +1782,10 @@ help:
 	@echo  '		3: more obscure warnings, can most likely be ignored'
 	@echo  '		e: warnings are being treated as errors'
 	@echo  '		Multiple levels can be combined with W=12 or W=123'
+	@$(if $(dtstree), \
+		echo '  make CHECK_DTBS=1 [targets] Check all generated dtb files against schema'; \
+		echo '         This can be applied both to "dtbs" and to individual "foo.dtb" targets' ; \
+		)
 	@echo  ''
 	@echo  'Execute "make" or "make all" to build all targets marked with [*] '
 	@echo  'For further info see the ./README file'