tools: kwboot: Calculate real used space in kwbimage header when calling kwboot_img_g...

tools: kwboot: Calculate real used space in kwbimage header when calling kwboot_img_grow_hdr()

Size of the header stored in kwbimage may be larger than real used size in
the kwbimage header. If there is unused space in kwbimage header then use
it for growing it. So update code to calculate used space of 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>