Tool for mounting USB sticks formatted with --statefuldev

sosa@'s statefuldev change, which installs all developer packages on
the stateful partition and has put /var there as well, means that we
can no longer just mount the rootfs on a usb stick and emerge packages
onto/off of the removeable device.  We now need to mount the stateful
partition as well, and also bindmount from /var on the rootfs to /var
on the stateful partition.  All portage state is stored there, among
other things.

Review URL: http://codereview.chromium.org/1644001

prep_usb.sh

@@ -0,0 +1,37 @@
+#!/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.
+
+# Load common constants.  This should be the first executable line.
+# The path to common.sh should be relative to your script's location.
+. "$(dirname "$0")/common.sh"
+
+DEFINE_boolean "unmount" $FLAGS_FALSE "unmount USB partitions" "u"
+DEFINE_string "device" "/dev/sdc" \
+    "The device on which the mountable partitions live." "d"
+DEFINE_string "rootfs_mountpt" "/tmp/m" "Mount point for rootfs" "r"
+DEFINE_string "stateful_mountpt" "/tmp/s" \
+    "Mount point for stateful partition" "s"
+
+# Parse command line.
+FLAGS "$@" || exit 1
+eval set -- "${FLAGS_ARGV}"
+
+mkdir -p "${FLAGS_rootfs_mountpt}"
+mkdir -p "${FLAGS_stateful_mountpt}"
+
+if [[ $FLAGS_unmount -eq $FLAGS_FALSE ]]; then
+  sudo mount "${FLAGS_device}3" "${FLAGS_rootfs_mountpt}"
+  sudo mount "${FLAGS_device}1" "${FLAGS_stateful_mountpt}"
+  sudo mount --bind "${FLAGS_stateful_mountpt}/var" \
+      "${FLAGS_rootfs_mountpt}/var"
+  echo "RootFS of bootable medium mounted at ${FLAGS_rootfs_mountpt}."
+else
+  echo "Attempting to unmount ${FLAGS_stateful_mountpt} " \
+      "and ${FLAGS_rootfs_mountpt}"
+  sudo umount "${FLAGS_rootfs_mountpt}/var"
+  sudo umount "${FLAGS_stateful_mountpt}"
+  sudo umount "${FLAGS_rootfs_mountpt}"
+fi