CMakeLists: add support for external overlays

Allow users who pull ArduinoCore as a module (via west) to provide
their own Zephyr overlay files in the application repository instead
of modifying ArduinoCore/variants.
This enables board-specific overlays (for example when using an
AtomS3 board) to live in the consuming project. If an overlay is not
provided the build will fail due to the missing zephyr,user section,
which is the expected behavior.

Signed-off-by: Michal Gagos <lizard123137@gmail.com>

Author: lizard123137

CMakeLists.txt

@@ -8,7 +8,11 @@ if (CONFIG_ARDUINO_API)
   elseif (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/variants/${NORMALIZED_BOARD_TARGET})
     set(variant_dir variants/${NORMALIZED_BOARD_TARGET})
   else()
-    message(FATAL_ERROR "Variant dir not found: variants/${BOARD}, variants/${NORMALIZED_BOARD_TARGET}")
+    message(WARNING
+      "Variant dir not found: variants/${BOARD}, variants/${NORMALIZED_BOARD_TARGET}\n"
+      "Your board is currently not supported. To use it you must create a DTS overlay."
+    )
+    set(variant_dir variants/default)
   endif()
 
   add_subdirectory(cores)

documentation/variants.md

@@ -38,6 +38,8 @@ variants/
 │   ├── variant.h
 ```
 
+It is possible to apply the overlay outside of the `variants` folder. In that case `cmake` will show a warning and a default empty `variant.h` file will be used. The `zephyr,user` section will still need to be defined somewhere in your project, as described in the [Guide to Writing Overlays section](#guide-to-writing-overlays).
+
 ## Guide to Writing Overlays
 
 ### DeviceTree Overlay files for Arduino boards

variants/default/variant.h

@@ -0,0 +1,7 @@
+/*
+ * Copyright (c) 2026 Michal Gagos
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#pragma once