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>
diff --git a/boot/bootutil/src/bootutil_misc.c b/boot/bootutil/src/bootutil_misc.c
index 0cfed0d..9787005 100644
--- a/boot/bootutil/src/bootutil_misc.c
+++ b/boot/bootutil/src/bootutil_misc.c
@@ -390,7 +390,7 @@
 
 uint32_t bootutil_max_image_size(const struct flash_area *fap)
 {
-#if defined(MCUBOOT_SWAP_USING_SCRATCH)
+#if defined(MCUBOOT_SWAP_USING_SCRATCH) || defined(MCUBOOT_SINGLE_APPLICATION_SLOT)
     return boot_status_off(fap);
 #elif defined(MCUBOOT_SWAP_USING_MOVE)
     struct flash_sector sector;