External overlays support

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