tools: kwbimage: Print size of binary header in kwbimage_print_header()

The binary header in kwbimage contains executable SPL code.

Print information about this binary header and not only information
about it's data part.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>