Fix external SDK path Linux queezythegreat#168

Author: Erriez

cmake/ArduinoToolchain.cmake

@@ -36,7 +36,7 @@ endif()
 #=============================================================================#
 #                         Detect Arduino SDK                                  #
 #=============================================================================#
-if(NOT ARDUINO_SDK_PATH)
+if(NOT DEFINED ARDUINO_SDK_PATH)
     set(ARDUINO_PATHS)
 
     foreach(DETECT_VERSION_MAJOR 1)
@@ -74,7 +74,7 @@ find_path(ARDUINO_SDK_PATH
           HINTS ${SDK_PATH_HINTS}
           DOC "Arduino SDK path.")
 
-if(ARDUINO_SDK_PATH)
+if(DEFINED ARDUINO_SDK_PATH)
     list(APPEND CMAKE_SYSTEM_PREFIX_PATH ${ARDUINO_SDK_PATH}/hardware/tools/avr)
     list(APPEND CMAKE_SYSTEM_PREFIX_PATH ${ARDUINO_SDK_PATH}/hardware/tools/avr/utils)
 else()