1. wpa_supplicatn use extra SHA default
2. mbedtls use extra SHA default
3. bootloader use extra SHA default
4. user code can use extra SHA default
Above all, using unified module is easy to maintenance code and save rom or ram.
* Separating MBEDTLS_SSL_MAX_CONTENT_LEN into input and output buffers.
* Beneficial for saving memory by reducing Tx buffer independently of Rx buffer.
* Taken from https://github.com/ARMmbed/mbedtls/pull/536