kbuild: take into account DT_SCHEMA_FILES changes while checking dtbs

kbuild: take into account DT_SCHEMA_FILES changes while checking dtbs

It is useful to be able to recheck dtbs files against a limited set of
DT schema files. This can be accomplished by using differnt
DT_SCHEMA_FILES argument values while rerunning make dtbs_check. However
for some reason if_changed_rule doesn't pick up the rule_dtc changes
(and doesn't retrigger the build).

Fix this by changing if_changed_rule to if_changed_dep and squashing DTC
and dt-validate into a single new command. Then if_changed_dep triggers
on DT_SCHEMA_FILES changes and reruns the build/check.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20220915114422.79378-1-dmitry.baryshkov@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>