[nrf fromtree] zephyr: Provide third image cases for direct image upload

The commit adds missing support for direct upload of third
image slots.

(cherry picked from commit 0035c33)
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

boot/zephyr/flash_map_extended.c

@@ -109,6 +109,14 @@ int flash_area_id_from_direct_image(int image_id)
 #if FIXED_PARTITION_EXISTS(slot3_partition)
     case 4:
         return FIXED_PARTITION_ID(slot3_partition);
+#endif
+#if FIXED_PARTITION_EXISTS(slot4_partition)
+    case 5:
+        return FIXED_PARTITION_ID(slot4_partition);
+#endif
+#if FIXED_PARTITION_EXISTS(slot5_partition)
+    case 6:
+        return FIXED_PARTITION_ID(slot5_partition);
 #endif
     }
     return -EINVAL;