Changes
   * config.h has been split into build_info.h and mbedtls_config.h
     build_info.h is intended to be included from C code directly, while
     mbedtls_config.h is intended to be edited by end users wishing to
     change the build configuration, and should generally only be included from
     build_info.h.
   * The handling of MBEDTLS_CONFIG_FILE has been moved into build_info.h.
   * Mandatory version symbols MBEDTLS_CONFIG_VERSION and
     MBEDTLS_USER_CONFIG_VERSION were introduced for MBEDTLS_CONFIG_FILE and
     MBEDTLS_USER_CONFIG_FILE respectively. They have to be defined with a
     value of one in their respective config file for the config to be
     considered valid.