Variant folder is only necessary on core-as-lib projects

Author: facchinm

Diff for: CMakeLists.txt

@@ -7,8 +7,10 @@ if (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/variants/${BOARD})
 elseif (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/variants/${NORMALIZED_BOARD_TARGET})
   set(variant_dir variants/${NORMALIZED_BOARD_TARGET})
 else()
+if (CONFIG_ARDUINO_API)
   message(FATAL_ERROR "Variant dir not found: variants/${BOARD}, variants/${NORMALIZED_BOARD_TARGET}")
 endif()
+endif()
 
 if (CONFIG_ARDUINO_API)
 add_subdirectory(cores)