Introduce a level of indirection and versioning in the config files ------------------------------------------------------------------- `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 wishing 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. A config file version symbol, `MBEDTLS_CONFIG_VERSION` was introduced. Defining it to a particular value will ensure that mbedtls interprets the config file in a way that's compatible with the config file format used by the mbedtls release whose `MBEDTLS_VERSION_NUMBER` has the same value. The only value supported by mbedtls 3.0.0 is `0x03000000`.