tools: kwboot: Do not modify kwbimage header before increasing its size

This ensures that kwboot_img_grow_hdr() function still sees valid kwbimage
header.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>