bootutil: Don't default to scratch when single app is used

When MCUBOOT_SINGLE_APPLICATION_SLOT is set then the app can
only be overwritten with new image and scratch algorithm
is not used. The configuration logic would default
MCUBOOT_SWAP_USING_SCRATCH to 1 because it was lacking
check for MCUBOOT_SINGLE_APPLICATION_SLOT.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>