boot: Add AES256 support for image encryption

Support only works when using mbedtls as the cryptographic library.

Signed-off-by: Salome Thirot <salome.thirot@arm.com>
diff --git a/boot/bootutil/src/caps.c b/boot/bootutil/src/caps.c
index a21bc09..87bac1a 100644
--- a/boot/bootutil/src/caps.c
+++ b/boot/bootutil/src/caps.c
@@ -2,6 +2,7 @@
  * SPDX-License-Identifier: Apache-2.0
  *
  * Copyright (c) 2017 Linaro Limited
+ * Copyright (c) 2021 Arm Limited
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -68,6 +69,9 @@
 #if defined(MCUBOOT_BOOTSTRAP)
     res |= BOOTUTIL_CAP_BOOTSTRAP;
 #endif
+#if defined(MCUBOOT_AES_256)
+    res |= BOOTUTIL_CAP_AES256;
+#endif
 
     return res;
 }