sim: Use non-equal security counters when testing upgrades

When testing upgrades, the simulator was always using two images having
the same security counter. This was preventing to test that the security
counters are updated at the right time in the scenarios where a revert
is possible. The upgrade image is now generated with a higher security
counter than the original image, enabling to detect e.g. the issue fixed
by the previous commit.

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