it is very important to check OUTPUT_DIR

Signed-off-by: carbon <carbon@milkv.io>

build.sh

@@ -81,7 +81,11 @@ function prepare_env()
   source build/${MV_BUILD_ENV} > /dev/null 2>&1
   defconfig ${MV_BOARD_LINK} > /dev/null 2>&1
 
-  echo "OUTPUT_DIR: ${OUTPUT_DIR}"  # @build/milkvsetup.sh
+  echo "OUTPUT_DIR: ${OUTPUT_DIR}"  # @build/envsetup_milkv.sh
+  if [ -z "${OUTPUT_DIR// }" ]; then
+    print_err "OUTPUT_DIR is not assigned, please check!"
+    exit 1
+  fi
 
   if [ "${STORAGE_TYPE}" == "sd" ]; then
     MILKV_IMAGE_CONFIG=device/${MILKV_BOARD}/genimage.cfg