tools: kwbimage: Fix calculating size of kwbimage v0 header

Extended and binary headers are optional and are part of the image header.

Fixes kwboot to determinate correct length of Dove images.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>