Provide means to reset handshake cert list

Extend mbedtls_ssl_set_hs_own_cert() to reset handshake cert list
if cert provided is null.  Previously, mbedtls_ssl_set_hs_own_cert()
only provided a way to append to the handshake certificate list,
without providing a way to replace the handshake certificate list.

Signed-off-by: Glenn Strauss <gstrauss@gluelogic.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 0e93849..d1fec95 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -3546,6 +3546,9 @@
  * \note           Same as \c mbedtls_ssl_conf_own_cert() but for use within
  *                 the SNI callback.
  *
+ * \note           Passing null \c own_cert clears the certificate list for
+ *                 the current handshake.
+ *
  * \param ssl      SSL context
  * \param own_cert own public certificate chain
  * \param pk_key   own private key