Generic hashing algorithm setup is already supported in PolarSSL/MbedTLS

It's supported [long time ago](https://github.com/ARMmbed/mbedtls/commit/1737385e04ef392927646ed939000436f2da2240), see hashing module [docs](https://tls.mbed.org/api/group__hashing__module.html) and [md_setup()](https://tls.mbed.org/api/md_8h.html#a1b858111212997b90bd7d2c71010a7ec).