[mbed-tls] Custom MBEDTLS_CONFIG_FILE with CMake subproject

Hello,

I need some help to setup mbedtls as a CMake subproject, I added it as a subdirectory in my CMakeLists. However, I can't figure out how to use my own config file for mbedtls (MBEDTLS_CONFIG_FILE option to the file I want as a config file).

When I put my custom config file to the include folder of mbedtls in the place of the original mbedtls_config.h file it works wel,l but it's not a future-proof option for me, is there anyone that could help me with this setup ?