variants: remove empty variant.h

There was an unnecessary dependency on having empty variant.h
files in each variant. Remove this if it's unused and use a
default empty one instead. This means we shall only require an
overlay when adding new platform support here.

update the documentation with this as well

Signed-off-by: Dhruva Gole <d-gole@ti.com>

Author: DhruvaG2000

CMakeLists.txt

@@ -3,15 +3,13 @@
 if (CONFIG_ARDUINO_API)
   string(REPLACE "__" "_" NORMALIZED_BOARD_TARGET "${NORMALIZED_BOARD_TARGET}")
 
-  if (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/variants/${BOARD})
+  # Find variant directory and check for variant.h
+  # If board-specific variant.h exists, use it; otherwise fall back to default
+  if (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/variants/${BOARD}/variant.h)
     set(variant_dir variants/${BOARD})
-  elseif (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/variants/${NORMALIZED_BOARD_TARGET})
+  elseif (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/variants/${NORMALIZED_BOARD_TARGET}/variant.h)
     set(variant_dir variants/${NORMALIZED_BOARD_TARGET})
   else()
-    message(WARNING
-      "Variant dir not found: variants/${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()