修改croot实现cout可以快速进入输出目录

在build/envsetup.sh中，参照croot()的实现，实现了新的cout功能，可以快速切换到产品输出目录。

function cout()

{

    T=$(gettop)

    if [ "$T" ]; then

        if [ "$ANDROID_PRODUCT_OUT" ]; then

            cd $ANDROID_PRODUCT_OUT

        else

            echo "Couldn't locate the out directory. Try use lunch first "

        fi

    else

        echo "Couldn't locate the top of the tree.  Try setting TOP."

    fi

}