UBI: add image sequence number to EC header

An image sequence number is added to the UBI erase-counter header
to be able determine if the root file system contains a mixture
of old and new images (because the flashing failed to complete).

A change to nolo is also needed for this to take effect.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>