CMakeLists.txt: Improve overlay search rules

[soburi]

The latest Zephyr update requires path separator insertion, so we've updated it accordingly.
We've also made changes to handle board revisions.

[Copilot]

Pull request overview

This PR updates how Arduino variants and devicetree overlays are located to align with recent Zephyr changes and to support per‑board revisions.

Changes:

• In the root CMakeLists.txt, normalize NORMALIZED_BOARD_TARGET and adjust variant_dir lookup to prefer variants/${BOARD} with a fallback to variants/${NORMALIZED_BOARD_TARGET}, improving compatibility with Zephyr’s updated board naming/path rules.
• In all Arduino sample CMakeLists.txt, change DTC_OVERLAY_FILE handling to first look for a ${NORMALIZED_BOARD_TARGET}_${BOARD_REVISION}.overlay and fall back to the base ${NORMALIZED_BOARD_TARGET}.overlay, so board revisions can use dedicated overlays.
• Keep behavior safe when no module-specific overlay exists by not forcing DTC_OVERLAY_FILE if neither the revision-specific nor the base overlay file is present.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
CMakeLists.txt	Normalizes NORMALIZED_BOARD_TARGET (replacing __ with _) and updates variant directory selection to try variants/${BOARD} first, then variants/${NORMALIZED_BOARD_TARGET}, with an updated fatal error message if neither exists.
samples/threads_arduino/CMakeLists.txt	Adds conditional logic to set DTC_OVERLAY_FILE to a board‑revision overlay if it exists, otherwise to the base overlay, for the threads Arduino sample.
samples/spi_controller/CMakeLists.txt	Same overlay selection logic as above, applied to the SPI controller sample.
samples/serial_event/CMakeLists.txt	Same overlay selection logic as above, applied to the serial event sample.
samples/i2cdemo/CMakeLists.txt	Same overlay selection logic as above, applied to the I²C demo sample.
samples/hello_arduino/CMakeLists.txt	Same overlay selection logic as above, applied to the hello Arduino sample.
samples/fade/CMakeLists.txt	Same overlay selection logic as above, applied to the fade sample.
samples/button_press_led/CMakeLists.txt	Same overlay selection logic as above, applied to the button‑press‑LED sample.
samples/blinky_arduino/CMakeLists.txt	Same overlay selection logic as above, applied to the blinky Arduino sample.
samples/attach_interrupt/CMakeLists.txt	Same overlay selection logic as above, applied to the attach‑interrupt sample.
samples/analog_input/CMakeLists.txt	Same overlay selection logic as above, applied to the analog input sample.

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