I work in Espressif Systems. I am currently working on providing an alternate hardware RSA sign implementation for mbedtls software sign for Espressif's new chip ESP32S2. I have gone through mbedTLS API in detail but I dont see any option where I can only replace mbedTLS software sign function with our hardware sign function . 
        I have gone through the issue https://tls.mbed.org/discussions/generic/using-an-external-rsa-private-key I have seen that there is a function named `mbedtls_pk_setup_rsa_alt` where we only register private key related function to the ALT_CTX which mbedtls uses to perform RSA sign. but this is not supported for TLS connections.
I have seen that there is MBEDTLS_RSA_ALT option in mbedTLS where we can provide alternate function to many of mbedTLS API, but we do not want to change any of the other functions, just provide alternate implementation of hardware sign. If we go with this way, there will be lot of duplicate code which will be needed to be maintained.
        can mbedTLS provide option to use mbedtls_rsa_alt context in its file `pkparse.c` so as to allow rsa sign using an extrnal private key.

Thanks and Regards,