tools: kwbimage: Fix checksum calculation for v1 images

Recent changes caused fields in the image main header to be modified
after the header checksum had already been computed. Move the checksum
computation to once again be the last operation performed on the header.

Fixes: 2b0980c2 ("tools: kwbimage: Fill the real header size into the main header")

Signed-off-by: Pierre Bourdon <delroth@gmail.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>