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/NOTICE b/NOTICE
index 0b3435b..ce203c1 100644
--- a/NOTICE
+++ b/NOTICE
@@ -8,4 +8,4 @@
 Runtime Inc, copyright 2015.
 
 Portions of this software were developed at
-Arm Limited, copyright 2019-2020.
+Arm Limited, copyright 2019-2021.