mbedtls: remove MBEDTLS_HAVE_TIME

When MbedTLS TLS features were added MBEDTLS_HAVE_TIME was defined as part
of enabling https:// support. However that pointed to the wrong function
which could crash if it received a NULL pointer.

Looking closer that function is not really needed, as it only seems to
increase the RNG entropy by using 4b of the current time and date.
The reason that was enabled is that lwIP was unconditionally requiring it,
although it's configurable and can be turned off.

Since lwIP doesn't use that field anywhere else, make it conditional and
disable it from our config.

Fixes: commit a564f5094f ("mbedtls: Enable TLS 1.2 support")
Reported-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>

lib/lwip/lwip/src/apps/altcp_tls/altcp_tls_mbedtls.c

@@ -692,7 +692,9 @@ altcp_tls_set_session(struct altcp_pcb *conn, struct altcp_tls_session *session)
   if (session && conn && conn->state) {
     altcp_mbedtls_state_t *state = (altcp_mbedtls_state_t *)conn->state;
     int ret = -1;
+#ifdef MBEDTLS_HAVE_TIME
     if (session->data.MBEDTLS_PRIVATE(start))
+#endif
       ret = mbedtls_ssl_set_session(&state->ssl_context, &session->data);
     return ret < 0 ? ERR_VAL : ERR_OK;
   }