kbuild: allow to run dt_binding_check and dtbs_check in a single command

Since commit 93512dad334d ("dt-bindings: Improve validation build error
handling"), 'make dtbs_check' does not validate the schema fully.

If you want to check everything, you need to run two commands separately.

  $ make ARCH=arm dt_binding_check
  $ make ARCH=arm dtbs_check

They are exclusive each other, so you cannot do like this:

  $ make ARCH=arm dt_binding_check dtbs_check

In this case, dt-doc-validate and dt-extract-example are skipped
because CHECK_DTBS is set.

Let's make it possible to run these two targets in a single command.
It will be useful for schema writers.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
diff --git a/Makefile b/Makefile
index da1966e..9387c96 100644
--- a/Makefile
+++ b/Makefile
@@ -1250,10 +1250,10 @@
 	$(Q)$(MAKE) $(build)=$(dtstree)
 
 ifneq ($(filter dtbs_check, $(MAKECMDGOALS)),)
+export CHECK_DTBS=y
 dtbs: dt_binding_check
 endif
 
-dtbs_check: export CHECK_DTBS=1
 dtbs_check: dtbs
 
 dtbs_install:
@@ -1269,6 +1269,10 @@
 scripts_dtc: scripts_basic
 	$(Q)$(MAKE) $(build)=scripts/dtc
 
+ifneq ($(filter dt_binding_check, $(MAKECMDGOALS)),)
+export CHECK_DT_BINDING=y
+endif
+
 PHONY += dt_binding_check
 dt_binding_check: scripts_dtc
 	$(Q)$(MAKE) $(build)=Documentation/devicetree/bindings