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>