add partition slection when backing up the original firmware

[inspireMeNow]

The current script backups up the firmware to the first partition in the USB drive by default. This commit will support back up the original firmware to the custom partition. I also found that when the USB drive has uneven partitions, the baking up process will be failure. For example, if /dev/sdc only have a partition named /dev/sdc4, the backing up process will be failure. This commit will fix it.

[MrChromebox]

so contrary to the PR/commit messages, this only adds selection of the partition when restoring the stock firmware from USB, not for when backing up

[inspireMeNow]

Sorry, I didn't see the comment correctly, I will add partition selection in backup and restore process later.

[inspireMeNow]

Now I have added the partition selection in backup process.