Handle VID header offset in ubi part command

The VID header offset is sometimes needed to initialize the UBI
partition. This patch adds it (optionally) to the command line
for the ubi part command.

(Lines have been properly wrapped since last version)

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>