zephyr: Fix trailer size computation for swap-scratch

When swap-scratch is used, the trailer size doesn't have to be rounded
up to the next multiple of the sector size. Indeed, the trailer only
need to be sector-aligned for swap-move and swap-offset.

Note: This PR has been modified from the upstream commit due to
a merge failure from other changes that are not being brought into
Zephyr 3.7

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>
(cherry picked from commit d5d3359e)