Update wipe to work with verified rootfs

client/site_tests/factory_Wipe/src/switch_partitions.sh

 #!/bin/sh
 # 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.
 
 # This script can be called to switch from kernel slot A / sda3
 # to kernel slot B / sda5 and vice versa.
 
-ROOT_DEV=$(rootdev)
+ROOT_DEV=$(rootdev -s)
 OTHER_ROOT_DEV=$(echo $ROOT_DEV | tr '35' '53')
 
 if [ "${ROOT_DEV}" = "${OTHER_ROOT_DEV}" ]
 then
   echo "Not a normal rootfs partition (3 or 5): ${ROOT_DEV}"
   exit 1
 fi
 
 # Successfully being able to mount the other partition
 # and run postinst guarantees that there is a real partition there.
 echo "Running postinst on $OTHER_ROOT_DEV"
-MOUNTPOINT=/tmp/newpart
+MOUNTPOINT=$(mktemp -d)
 mkdir -p "$MOUNTPOINT"
-mount "$OTHER_ROOT_DEV" "$MOUNTPOINT"
+mount -o ro  "$OTHER_ROOT_DEV" "$MOUNTPOINT"
 "$MOUNTPOINT"/postinst "$OTHER_ROOT_DEV"
 POSTINST_RETURN_CODE=$?
 umount "$MOUNTPOINT"
 rmdir "$MOUNTPOINT"
 
 exit $POSTINST_RETURN_CODE