boot: bootutil: Take into account scratch trailer when computing max image size

When using swap-scratch and the trailer is larger than a single sector,
the sector containing the last part of the firmware data might only
contain a very small part of the trailer and some padding might be
necessary between the end of the firmware image and the beginning of the
trailer to ensure the scratch trailer won't be overwritten when copying
that sector to the scratch area.

This commit updates the 'bootutil_max_image_size' routine to take that
padding into account.

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