MBEDTLS_PLATFORM_H macro

\file platform.h This file contains the definitions and functions of the Mbed TLS platform abstraction layer. The platform abstraction layer removes the need for the library to directly link to standard C library functions or operating system services, making the library easier to port and embed. Application developers and users of the library can provide their own implementations of these functions, or implementations specific to their platform, which can be statically linked to the library or dynamically configured at runtime. When all compilation options related to platform abstraction are disabled, this header just defines `mbedtls_xxx` function names as aliases to the standard `xxx` function. Most modules in the library and example programs are expected to include this header.