tools: kwboot: Explicitly check against size of struct main_hdr_v1

Explicitly check the image size against size of struct main_hdr_v1.
This way the check is more readable, since the `hdrsz` variable
may semantically contain another value.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
1 file changed