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>