tools: kwbimage: Fix generating of kwbimage v0 header checksum

Checksum for v0 image must be generated after filling all fields in the
main header. Otherwise it would be invalid.

Exactly same problem for v1 images was already fixed in the past in commit
9203c738 ("tools: kwbimage: Fix checksum calculation for v1 images").

Fixes: 5c61710c ("tools: kwbimage: Properly set srcaddr in kwbimage v0")
Signed-off-by: Pali Rohár <pali@kernel.org>