boot: zephyr: prefer swap move if scratch_partition not enabled

With the exception of nrf targets BOOT_SWAP_USING_SCRATCH mode was
still the default algorithm.

Changing the preferred mode in cases where no scratch_partition is
defined will allow successfully building mcuboot for such boards w/o
the need for any board specific overlays.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>