Allow upgrades of only one slot
Allow upgrades of only one slot

When only upgrading one slot, the swap type for the non upgraded slot is
BOOT_SWAP_TYPE_FAIL instead of NONE.  Account for this, and check slot 0
in this case.

This is probably not the correct solution, but it is unclear of the
timing of when the dependencies of the other images first slot would be
checked.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>
Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

tamban01/mcuboot,utzig/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,utzig/mcuboot,ATmobica/mcuboot