Allow Ant to use the existing NDK path variable inline

“ndk.dir” has to be manually set in the local.properties file
“android update project” only covers auto-import of the sdk

shell/android/build.xml

@@ -36,6 +36,9 @@
     <condition property="sdk.dir" value="${env.ANDROID_HOME}">
         <isset property="env.ANDROID_HOME" />
     </condition>
+    <condition property="ndk.dir" value="${env.ANDROID_NDK}">
+        <isset property="env.ANDROID_NDK" />
+    </condition>
 
     <!-- The project.properties file is created and updated by the 'android'
          tool, as well as ADT.