sim: Use image number instead of slot in dep test

When generating dependencies, we were using the slot number instead of
the image number to determine which dependencies to mark invalid.  Fix
this, so that we can test configurations where one image is upgradeable
and the other not.

Signed-off-by: David Brown <david.brown@linaro.org>
Signed-off-by: Fabio Utzig <utzig@apache.org>