sim: fix encryption for ram-load related tests

If ram-load is being used with encryption and
the higher version image is loaded from the primary slot the
verification will fail as the image is always non-encrypted
and will produce an invalid hash. This fix puts encrypted images
into both slots to prevent this issue.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I4ac9f332434a95d140c5572096b8a9161db2d217