Add the warning tag to the missing build board msg

Signed-off-by: Matteo Suppo <matteo.suppo@gmail.com>

Author: matteosuppo

src/arduino.cc/builder/constants/constants.go

@@ -218,7 +218,7 @@ const MSG_LIBRARIES_USED = " Used: {0}"
 const MSG_LIBRARY_CAN_USE_SRC_AND_UTILITY_FOLDERS = "Library can't use both 'src' and 'utility' folders. Double check {0}"
 const MSG_LIBRARY_INCOMPATIBLE_ARCH = "WARNING: library {0} claims to run on {1} architecture(s) and may be incompatible with your current board which runs on {2} architecture(s)."
 const MSG_LOOKING_FOR_RECIPES = "Looking for recipes like {0}*{1}"
-const MSG_MISSING_BUILD_BOARD = "Board {0}:{1}:{2} doesn''t define a ''build.board'' preference. Auto-set to: {3}"
+const MSG_MISSING_BUILD_BOARD = "Warning: Board {0}:{1}:{2} doesn''t define a ''build.board'' preference. Auto-set to: {3}"
 const MSG_MISSING_CORE_FOR_BOARD = "Selected board depends on '{0}' core (not installed)."
 const MSG_MUST_BE_A_FOLDER = "{0} must be a folder"
 const MSG_PACKAGE_UNKNOWN = "{0}: Unknown package"