API changes
   * mbedtls_ssl_conf_export_keys_ext_cb() and
     mbedtls_ssl_conf_export_keys_cb() have been removed
     and replaced by a new API
     mbedtls_ssl_set_export_keys_cb().
   * The signature of key export callbacks configured via
     mbedtls_ssl_set_export_keys_cb() is different from that
     of the previous mbedtls_ssl_conf_export_keys_cb(): First,
     raw keys and IVs are no longer exported. Further, callbacks
     now receive an additional parameter indicating the type
     of secret that's being exported, paving the way for the
     larger number of secrets in TLS 1.3. Finally, the key export
     callback and context are now connection-specific.

API changes
   * mbedtls_ssl_conf_export_keys_ext_cb() and
     mbedtls_ssl_conf_export_keys_cb() have been removed and
     replaced by a new API mbedtls_ssl_set_export_keys_cb().
     Raw keys and IVs are no longer passed to the callback.
     Further, callbacks now receive an additional parameter
     indicating the type of secret that's being exported,
     paving the way for the larger number of secrets
     in TLS 1.3. Finally, the key export callback and
     context are now connection-specific.

Improve ChangeLog wording for key export
Improve ChangeLog wording for key export

Signed-off-by: Hanno Becker <4d488caff78d6184f59bbe6e137e34d4606b9fe8@arm.com>
