SSL key export interface change

This affects users of the SSL key export APIs:

    mbedtls_ssl_conf_export_keys_cb()
    mbedtls_ssl_conf_export_keys_ext_cb()

Those APIs have been removed and replaced by the new API mbedtls_ssl_set_export_keys_cb(). This API differs from the previous key export API in the following ways:

• It is no longer bound to an SSL configuration, but to an SSL context. This allows users to more easily identify the connection an exported key belongs to.
• It no longer exports raw keys and IV.
• A secret type parameter has been added to identify which key is being exported. For TLS 1.2, only the master secret is exported, but upcoming TLS 1.3 support will add other kinds of keys.
• The callback now specifies a void return type, rather than returning an error code. It is the responsibility of the application to handle failures in the key export callback, for example by shutting down the TLS connection.

For users which do not rely on raw keys and IV, adjusting to the new callback type should be straightforward - see the example programs programs/ssl/ssl_client2 and programs/ssl/ssl_server2 for callbacks for NSSKeylog, EAP-TLS and DTLS-SRTP.

Users which require access to the raw keys used to secure application traffic may derive those by hand based on the master secret and the handshake transcript hashes which can be obtained from the raw data on the wire. Such users are also encouraged to reach out to the Mbed TLS team on the mailing list, to let the team know about their use case.