kbuild: Create a rule for building device tree overlay objects

This allows to handle device tree overlays like plain device trees.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
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v2:
  - Since commit 4f0e3a57d6eb727c ("kbuild: Add support for DT binding
    schema checks"), cmd_dtc takes a second parameter specifying the
    output format.
1 file changed