boot: enable partial multi-image booting

Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index 6102f3e..af62392 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -224,6 +224,7 @@
 
 #if (BOOT_IMAGE_NUMBER > 1)
     uint8_t curr_img_idx;
+    bool img_mask[BOOT_IMAGE_NUMBER];
 #endif
 
 #if defined(MCUBOOT_DIRECT_XIP) || defined(MCUBOOT_RAM_LOAD)