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/enc-aes256kw.b64 b/enc-aes256kw.b64
new file mode 100644
index 0000000..866aa6d
--- /dev/null
+++ b/enc-aes256kw.b64
@@ -0,0 +1 @@
+5FxRRtIcgjXMGhmvofKqIMiMf0Bs2yKqarXLqvixW7Q=