LinearLayout增加边框

下面是我的测试代码，最终效果LinearLayout有灰色边框，要怎样保留底部或顶部的边框，其它三个方向的边框去掉？
boder.xml


<?xml version= "1.0"  encoding= "UTF-8" ?>

<layer-list xmlns:android= "http://schemas.android.com/apk/res/android"  >

 

     <!-- 连框颜色值 -->

     <item>

         <shape>

             <solid android:color= "#000000"  />

         </shape>

     </item>

     <!-- 主体背景颜色值 -->

     <item

         android:top= "1dp"

         android:left= "1dp"

         android:right= "1dp" >

         <shape>

             <solid android:color= "#ffffff"  />

         </shape>

     </item>

</layer-list>






<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:orientation="vertical"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent" >


    <LinearLayout android:layout_width="fill_parent" android:layout_height="wrap_content"
        android:orientation="vertical" android:gravity="center"
        android:background="@drawable/boder" >      
        <TextView android:layout_width="fill_parent" android:layout_height="wrap_content"
            android:text="@string/log_name"/>         
    </LinearLayout>
 </LinearLayout>