Umount the device first during mount_usb_storage() (Bugfix) (#1587)

To prevent the inserted storage from failing to mount twice, first ensure it is unmounted.

Because the OS will auto-mount the inserted storage, the existing subprocess.call(["mount", device_to_mount, FOLDER_TO_MOUNT]) command will cause an error if the format of inserted storage is NTFS.

Perform an umount on device_to_mount (e.g., for the media card in #1587, it would be umount /dev/mmcblk0p1) before attempting to mount it again to avoid this issue.

checkbox-support/checkbox_support/scripts/usb_read_write.py

@@ -170,6 +170,8 @@ def mount_usb_storage(partition):
         # use pipe so I could hide message like
         # "umount: /tmp/tmpjzwb6lys: not mounted"
         subprocess.call(["umount", FOLDER_TO_MOUNT], stderr=subprocess.PIPE)
+        # umount device_to_mount such as "umount /dev/mmcblk0p1" to avoid when the storage type is ntfs, which could not be mounted twice.
+        subprocess.call(["umount", device_to_mount], stderr=subprocess.PIPE)
         # mount the target device/partition
         # if the return code of the shell command is non-zero,
         # means something wrong.