kwbimage: align v1 binary header to 4B

According to the Armada-XP documentation the binary header format
requires the header length to be aligned to 4B.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>