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.

library/psa_crypto.c

@@ -0,0 +1,97 @@
+/*
+ *  PSA crypto layer on top of Mbed TLS crypto
+ */
+/*  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)
+ */
+
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
+#if defined(MBEDTLS_PSA_CRYPTO_C)
+
+#include "psa/crypto.h"
+
+#include "mbedtls/ctr_drbg.h"
+#include "mbedtls/entropy.h"
+
+
+/* Implementation that should never be optimized out by the compiler */
+static void mbedtls_zeroize( void *v, size_t n )
+{
+    volatile unsigned char *p = v; while( n-- ) *p++ = 0;
+}
+
+typedef struct {
+    int initialized;
+    mbedtls_entropy_context entropy;
+    mbedtls_ctr_drbg_context ctr_drbg;
+} psa_global_data_t;
+
+static psa_global_data_t global_data;
+
+static psa_status_t mbedtls_to_psa_error( int ret )
+{
+    switch( ret )
+    {
+        case 0:
+            return( PSA_SUCCESS );
+        case MBEDTLS_ERR_ENTROPY_NO_SOURCES_DEFINED:
+        case MBEDTLS_ERR_ENTROPY_NO_STRONG_SOURCE:
+        case MBEDTLS_ERR_ENTROPY_SOURCE_FAILED:
+            return( PSA_ERROR_INSUFFICIENT_ENTROPY );
+        default:
+            return( PSA_ERROR_UNKNOWN_ERROR );
+    }
+}
+
+void mbedtls_psa_crypto_free( void )
+{
+    mbedtls_ctr_drbg_free( &global_data.ctr_drbg );
+    mbedtls_entropy_free( &global_data.entropy );
+    mbedtls_zeroize( &global_data, sizeof( global_data ) );
+}
+
+psa_status_t psa_crypto_init( void )
+{
+    int ret;
+    const unsigned char drbg_seed[] = "PSA";
+
+    if( global_data.initialized != 0 )
+        return( PSA_SUCCESS );
+
+    mbedtls_zeroize( &global_data, sizeof( global_data ) );
+    mbedtls_entropy_init( &global_data.entropy );
+    mbedtls_ctr_drbg_init( &global_data.ctr_drbg );
+
+    ret = mbedtls_ctr_drbg_seed( &global_data.ctr_drbg,
+                                 mbedtls_entropy_func,
+                                 &global_data.entropy,
+                                 drbg_seed, sizeof( drbg_seed ) - 1 );
+    if( ret != 0 )
+        goto exit;
+
+exit:
+    if( ret != 0 )
+        mbedtls_psa_crypto_free( );
+    return( mbedtls_to_psa_error( ret ) );
+}
+
+#endif /* MBEDTLS_PSA_CRYPTO_C */