Add more safeguards to image_to_usb.sh. List USB drives if --to is omitted

src/platform/installer/chromeos-common.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 contains common constants and functions for installer scripts. This must
 # evaluate properly for both /bin/bash and /bin/sh, since it's used both to
 # create the initial image at compile time and to install or upgrade a running
 # image.
 
 # Here are the GUIDs we'll be using to identify various partitions. NOTE: The
@@ skipping 492 matching lines @@
 # The scripts that source this file typically want to use the root password as
 # confirmation, unless the --run_as_root flag is given.
 dont_run_as_root() {
   if [ $(id -u) -eq "0" -a "${FLAGS_run_as_root}" -eq "${FLAGS_FALSE}" ]
   then
     echo "Note: You must be the 'chronos' user to run this script. Unless"
     echo "you pass --run_as_root and run as root."
     exit 1
   fi
 }
+
+list_usb_disks() {
+  local sd
+  for sd in /sys/block/sd*; do
+    if readlink ${sd}/device | grep -q usb &&
+      [ "$(cat ${sd}/removable)" = 1 ]; then
+      echo ${sd##*/}
+    fi
+  done
+}
+
+get_disk_info() {
+  # look for a "given" file somewhere in the path upwards from the device
+  local dev_path=/sys/block/${1}/device
+  while [ -d "${dev_path}" -a "${dev_path}" != "/sys" ]; do
+    if [ -f "${dev_path}/${2}" ]; then
+      cat "${dev_path}/${2}"
+      return
+    fi
+    dev_path=$(readlink -f ${dev_path}/..)
+  done
+  echo '[Unknown]'
+}