tools: kwbimage: Factor out add_binary_header_v1

In preparation of adding the creation of secure headers, we factor the
add_binary_header_v1 function out of the image_create_v1 function.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>