Introduce MBEDTLS_PRIVATE macro.

Public structs members are considered private and should not
be used by users application.

MBEDTLS_PRIVATE(member) macro is intended to clearly indicate
which members are private.

Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>

include/mbedtls/private_access.h

@@ -0,0 +1,32 @@
+ /**
+ * \file private_access.h
+ *
+ * \brief Macro wrapper for struct's memebrs.
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  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.
+ */
+
+#ifndef MBEDTLS_PRIVATE_ACCESS_H
+#define MBEDTLS_PRIVATE_ACCESS_H
+
+#ifndef MBEDTLS_ALLOW_PRIVATE_ACCESS
+#define MBEDTLS_PRIVATE(member) private_##member
+#else
+#define MBEDTLS_PRIVATE(member) member
+#endif
+
+#endif /* MBEDTLS_PRIVATE_ACCESS_H */