Allow larger primary slot in swap-move

When using swap in move mode, the current compatibility test only allows
for same size and layout between slots. This results in one sector that
can never be used in the secondary slot, which corresponds to the first
move up operation in the primary slot. This relaxes and checking a bit,
so both same size slots and a primary with one extra sector are valid.

Signed-off-by: Fabio Utzig <utzig@apache.org>