Add PSA crypto module

New module psa_crypto.c (MBEDTLS_PSA_CRYPTO_C):
Platform Security Architecture compatibility layer on top of
libmedcrypto.

Implement psa_crypto_init function which sets up a RNG.

Add a mbedtls_psa_crypto_free function which deinitializes the
library.

Define a first batch of error codes.

include/psa/crypto_extra.h

@@ -0,0 +1,46 @@
+/**
+ * \file psa/crypto_extra.h
+ *
+ * \brief PSA cryptography module: Mbed TLS vendor extensions
+ */
+/*
+ *  Copyright (C) 2018, ARM Limited, All Rights Reserved
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+
+#ifndef PSA_CRYPTO_EXTRA_H
+#define PSA_CRYPTO_EXTRA_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * \brief Library deinitialization.
+ *
+ * This function clears all data associated with the PSA layer,
+ * including the whole key store.
+ *
+ * This is an Mbed TLS extension.
+ */
+void mbedtls_psa_crypto_free( void );
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* PSA_CRYPTO_EXTRA_H */