boot: zephyr: add config for swap without scratch

Add Zephyr option to enable building a bootloader that uses an
alternative swap algorithm, that first moves up all sectors in slot1 and
then directly swaps between slot0 and slot1.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index 47da3b7..c50a1e9 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -58,7 +58,9 @@
     switch (slot) {
     case 0: return FLASH_AREA_IMAGE_PRIMARY(image_index);
     case 1: return FLASH_AREA_IMAGE_SECONDARY(image_index);
+#if !defined(CONFIG_BOOT_SWAP_USING_MOVE)
     case 2: return FLASH_AREA_IMAGE_SCRATCH;
+#endif
     }
 
     return -EINVAL; /* flash_area_open will fail on that */