We used to write the second firmware right after the first
firmware. However, splitting the available space into two
equal regions has advantages: When we update barebox the next
time the generated FCB/DBBT blocks will be the same as with the
last update, so in case of power failure it is more likely that
we have valid data in one of the FCB/DBBT areas.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>