Use new grub2 variables to boot from the device with the bootloader

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 468 matching lines @@
   if [ -x "$MKIMAGE" ]; then
     MBR_SCRIPT_UIMG="${MBR_SCRIPT}.uimg"
     "$MKIMAGE" -A "arm" -O linux  -T script -a 0 -e 0 -n "COS boot" \
                -d ${MBR_SCRIPT} ${MBR_SCRIPT_UIMG} >&2
   else
     echo "Error: u-boot mkimage not found or not executable." >&2
     exit 1
   fi
   echo ${MBR_SCRIPT_UIMG}
 }
+
+
+# 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
+}