Refuse to always clobber.

BUG=
TEST=

Review URL: http://codereview.chromium.org/8624003

git-svn-id: http://src.chromium.org/svn/trunk/src/build@111017 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

android/buildbot.sh

@@ -31,7 +31,7 @@ for mandatory_directory in "${ANDROID_SDK_ROOT}" "${ANDROID_NDK_ROOT}" ; do
   fi
 done
 
-if [ ! -n "$BUILDBOT_CLOBBER" ]; then
+if [ ! "$BUILDBOT_CLOBBER" = "" ]; then
   NEED_CLOBBER=1
 fi