zephyr: Provide third image cases for direct image upload

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

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index be90a8e..64e8008 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -110,6 +110,14 @@
     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;
 }