config.h was 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 whishing to change the build configuration, and should generally only be included from build_info.h. This is because all the preprocessor logic has been moved into build_info.h, including the handling of the MBEDTLS_CONFIG_FILE macro.
Mandatory version symbols were introduced for MBEDTLS_CONFIG_FILE and MBEDTLS_USER_CONFIG_FILE, MBEDTLS_CONFIG_VERSION and MBEDTLS_USER_CONFIG_VERSION respectively. Both config files should include a definiton of their respective version symbol, with a value of 1 to be considered valid.