sim: Fix largest image computation for swap-move and swap-offset

For the swap-move and swap-offset strategies, the computation of the
largest image size was not taking taking into account the padding that
is needed when using those strategies. Due to this limitation, the
simulator is currently using hardcoded image sizes, smaller than the
maximum possible size, when running tests for the swap-move or
swap-offset strategies. This commit fixes the maximum image size
computation for those strategies.

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