ubiformat: Dont ignore sequence number CLI option

Image sequence number for the UBI header can be specified for the
ubiformat tool according to the documentation and the help message for
ubiformat. The CLI option --image-seq for image sequence number is not
supported. -Q option for image sequence number is silently ignored.
This patch adds the CLI support for image sequence number.

Signed-off-by: Amol Vengurlekar <amol.sven@gmail.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>