ubiformat/ubinize: Don't randomize 0xFFFFFFFF sequence number

ubiformat/ubinize: Don't randomize 0xFFFFFFFF sequence number

args.image_seq is of type uint32_t and was initialized to -1 which
becomes 0xFFFFFFFF in this case. Later the value -1 was used as a flag
that args.image_seq should be replaced with a randomized value.

With the option --image-seq (-Q) a user could provide any sequence
number at will.

But when the user provided sequence number was 0xFFFFFFFF this was
understood effectivly as -1 and got overridden by a randomized
sequence number.

So this patch change the programm flow to respect the principle of
least surprise and never use a randomized sequence number when the
user provide one at own will.

Signed-off-by: Michael Roth <mroth@nessie.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>