kbuild: verify dtoverlay files against schema

Currently only the single part device trees are validated against DT
schema. For the multipart DT files only the base DTB is validated.
Extend the fdtoverlay commands to validate the resulting DTB file
against schema.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20240527-dtbo-check-schema-v1-1-ee1094f88f74@linaro.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>