tools: kwbimage: Refactor kwbimage header size determination

tools: kwbimage: Refactor kwbimage header size determination

Add functions kwbheader_size() and kwbheader_size_for_csum().

Refactor code determining header size to use these functions.

Refactor header checksum determining function.

Remove stuff that is not needed anymore.

This simplifies the code a little and fixes one instance of validating
header size meant for checksum instead of whole header size.

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