bootutil: Fix the reading of image headers after partial swap completion

After a partial swap has been resumed and completed, the image headers
are reloaded. The idea is that we want to update the bootloader state
(boot_data) to properly reflect the new state of the slots: the image
headers in the primary and secondary slots are now respectively the
headers of the new and previous active image.

However, the implementation was doing the exact opposite when
swap-scratch was used, which could lead to erroneous behaviors such as a
failure when validating the primary slot.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>