tools: kwbimage: Fix invalid secure boot header signature

Secure boot header signature is calculated from the image header with
zeroed header checksum. Calculation is done in add_secure_header_v1()
function. So after calling this function no header member except
main_hdr->checksum can be modified. Commit 2b0980c2 ("tools: kwbimage:
Fill the real header size into the main header") broke this requirement as
final header size started to be filled into main_hdr->headersz_* members
after the add_secure_header_v1() call.

Fix this issue by following steps:
- Split header size and image data offset into two variables (headersz and
  *dataoff).
- Change image_headersz_v0() and add_binary_header_v1() functions to return
  real (unaligned) header size instead of image data offset.
- On every place use correct variable (headersz or *dataoff)

After these steps variable headersz is correctly filled into the
main_hdr->headersz_* members and so overwriting them in the end of the
image_create_v1() function is not needed anymore. Remove those overwriting
which effectively reverts changes in problematic commit without affecting
value in main_hdr->headersz_* members and makes secure boot header
signature valid again.

Fixes: 2b0980c2 ("tools: kwbimage: Fill the real header size into the main header")
Signed-off-by: Pali Rohár <pali@kernel.org>