Use delta_generator from chroot instead of from update_engine directory.

cros_generate_update_payload

 #!/bin/bash
 
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Script to generate a Chromium OS update for use by the update engine.
 # If a source .bin is specified, the update is assumed to be a delta update.
 
 # Load common constants.  This should be the first executable line.
@@ skipping 150 matching lines @@
     echo "Generating an old-style full update"
   else
     echo "Generating a new-style full update"
   fi
 fi
 
 if [ "$DELTA" -eq "$FLAGS_TRUE" -o "$FLAGS_old_style" -eq "$FLAGS_FALSE" ]; then
   echo "Generating a delta update"
 
   # Sanity check that the real generator exists:
-  GENERATOR="$(dirname "$0")/../platform/update_engine/delta_generator"
-  [ -x "$GENERATOR" ] || die "$GENERATOR doesn't exist, or isn't executable"
+  GENERATOR="$(which delta_generator)"
+  [ -x "$GENERATOR" ] || die "can't find delta_generator"
 
   trap cleanup INT TERM EXIT
   if [ "$DELTA" -eq "$FLAGS_TRUE" ]; then
     if [ "$FLAGS_full_kernel" -eq "$FLAGS_FALSE" ]; then
       SRC_KERNEL=$(extract_partition_to_temp_file "$FLAGS_src_image" 2)
       if [ "$FLAGS_patch_kernel" -eq "$FLAGS_TRUE" ]; then
         patch_kernel "$FLAGS_src_image" "$SRC_KERNEL"
       fi
       echo md5sum of src kernel:
       md5sum "$SRC_KERNEL"
@@ skipping 49 matching lines @@
 
   GENERATOR="$(dirname "$0")/mk_memento_images.sh"
 
   CROS_GENERATE_UPDATE_PAYLOAD_CALLED=1 "$GENERATOR" "$DST_KERNEL" "$DST_ROOT"
   mv "$(dirname "$DST_KERNEL")/update.gz" "$FLAGS_output"
 
   trap - INT TERM EXIT
   cleanup noexit
   echo "Done generating full update."
 fi