crosutils: fix "umount: /tmp/XXX: not mounted" in build_image

update_bootloaders.sh

 #!/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.
 
 # Helper script that generates the legacy/efi bootloader partitions.
 # It does not populate the templates, but can update a loop device.
 
 . "$(dirname "$0")/common.sh"
@@ skipping 162 matching lines @@
   update_x86_bootloaders "${old_root}" \
                          "${kernel_cfg}" \
                          "${ESP_FS_DIR}" \
                          "${FLAGS_from}"
 
   # Install the syslinux loader on the ESP image (part 12) so it is ready when
   # we cut over from rootfs booting (extlinux).
   if [[ ${FLAGS_install_syslinux} -eq ${FLAGS_TRUE} ]]; then
     sudo umount "${ESP_FS_DIR}"
     sudo syslinux -d /syslinux "${ESP_DEV}"
+    # mount again for cleanup to free resource gracefully
+    sudo mount -o ro "${ESP_DEV}" "${ESP_FS_DIR}"
   fi
 elif [[ "${FLAGS_arch}" = "arm" ]]; then
   # Copy u-boot script to ESP partition
   if [ -r "${FLAGS_from}/boot-A.scr.uimg" ]; then
     sudo mkdir -p "${ESP_FS_DIR}/u-boot"
     sudo cp "${FLAGS_from}/boot-A.scr.uimg" \
       "${ESP_FS_DIR}/u-boot/boot.scr.uimg"
   fi
 fi
 
 set +e