Introduce a level of indirection in config header

Create a separate header file (mbedtls/build_info.h) to use when
depending on the config options defined in config.h.

Also copy the handling of the MBEDTLS_CONFIG_FILE macro into the new
header, so that the next commit can remove this code from every other
place where config.h used to be included.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

include/mbedtls/build_info.h

@@ -0,0 +1,35 @@
+/**
+ * \file build_info.h
+ *
+ * \brief Build-time configuration info
+ *
+ *  Include this file if you need to depend on the
+ *  configuration options defined in config.h
+ */
+ /*
+  *  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_BUILD_INFO_H
+#define MBEDTLS_BUILD_INFO_H
+
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
+#endif /* MBEDTLS_BUILD_INFO_H */