add default scripts

engine/Scripts/lib_build_android.sh

+engine_path=$(pwd)
+runtime_mode=release
+
+while getopts ":r:m:" opt
+do
+    case $opt in
+        r)
+        engine_path=$OPTARG # set engine_path, depot_tools and flutter engine folder will be put into this path
+        ;;
+        m)
+        runtime_mode=$OPTARG
+        ;;
+        ?)
+        echo "unknown param"
+        exit 1;;
+    esac
+done
+
+python3 lib_build.py -r $engine_path -p android -m $runtime_mode

engine/Scripts/lib_build_ios.sh

+engine_path=$(pwd)
+runtime_mode=release
+bitcode=""
+
+while getopts ":r:m:e" opt
+do
+    case $opt in
+        r)
+        engine_path=$OPTARG # set engine_path, depot_tools and flutter engine folder will be put into this path
+        ;;
+        m)
+        runtime_mode=$OPTARG
+        ;;
+        e)
+        bitcode="-e" # enable-bitcode switch
+        ;;
+        ?)
+        echo "unknown param"
+        exit 1;;
+    esac
+done
+
+python3 lib_build.py -r $engine_path -p ios -m $runtime_mode $bitcode
+

engine/Scripts/lib_build_mac.sh

+engine_path=$(pwd)
+runtime_mode=release
+
+while getopts ":r:m:" opt
+do
+    case $opt in
+        r)
+        engine_path=$OPTARG # set engine_path, depot_tools and flutter engine folder will be put into this path
+        ;;
+        m)
+        runtime_mode=$OPTARG
+        ;;
+        ?)
+        echo "unknown param"
+        exit 1;;
+    esac
+done
+
+python3 lib_build.py -r $engine_path -p mac -m $runtime_mode
+