arm: don't truncate the mbr when installing the boot script. bad things happen otherwise

src/scripts/build_image

 #!/bin/bash
 
 # Copyright (c) 2009 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 build a bootable keyfob-based chromeos system image from within
 # a chromiumos setup. This assumes that all needed packages have been built into
 # the given target's root with binary packages turned on. This script will
 # build the Chrome OS image using only pre-built binary packages.
@@ skipping 361 matching lines @@
           "bootm C0008000" > ${MBR_SCRIPT}
   MKIMAGE="${BOARD_ROOT}/u-boot/mkimage"
   if [[ -f "$MKIMAGE".gz ]]; then
     sudo gunzip "$MKIMAGE".gz
   fi
   if [[ -x "$MKIMAGE" ]]; then
     MBR_SCRIPT_UIMG="${MBR_SCRIPT}.uimg"
     "$MKIMAGE" -A "${ARCH}" -O linux  -T script -a 0 -e 0 -n "COS boot" \
                -d ${MBR_SCRIPT} ${MBR_SCRIPT_UIMG}
     dd bs=1 count=`stat --printf="%s" ${MBR_SCRIPT_UIMG}` \
-       if="$MBR_SCRIPT_UIMG" of="$MBR_IMG"
+       if="$MBR_SCRIPT_UIMG" of="$MBR_IMG" conv=notrunc
     hexdump -v -C "$MBR_IMG"
   else
     echo "Error: u-boot mkimage not found or not executable."
   fi
 fi
 
 OUTSIDE_OUTPUT_DIR="../build/images/${FLAGS_board}/${IMAGE_SUBDIR}"
 echo "Done.  Image created in ${OUTPUT_DIR}"
 echo "To copy to USB keyfob, OUTSIDE the chroot, do something like:"
 echo "  ./image_to_usb.sh --from=${OUTSIDE_OUTPUT_DIR} --to=/dev/sdb"
 echo "To convert to VMWare image, OUTSIDE the chroot, do something like:"
 echo "  ./image_to_vmware.sh --from=${OUTSIDE_OUTPUT_DIR}"
 echo "from the scripts directory where you entered the chroot."
 
 trap - EXIT