Allow compile-time configuration of timer callbacks

Introduces
- MBEDTLS_SSL_CONF_SET_TIMER
- MBEDTLS_SSL_CONF_GET_TIMER
which allows to configure timer callbacks at compile-time.

Impact on code-size:

|  | GCC 8.2.1 | ARMC5 5.06 | ARMC6 6.12 |
| --- | --- | --- | --- |
| `libmbedtls.a` before | 23379 | 23981 | 26941 |
| `libmbedtls.a` after | 23351 | 23953 | 26869 |
| gain in Bytes | 28 | 28 | 72 |
diff --git a/programs/ssl/dtls_client.c b/programs/ssl/dtls_client.c
index 6f508ae..700e197 100644
--- a/programs/ssl/dtls_client.c
+++ b/programs/ssl/dtls_client.c
@@ -213,8 +213,15 @@
      mbedtls_ssl_set_bio_ctx( &ssl, &server_fd );
 #endif
 
+#if defined(MBEDTLS_TIMING_C)
+#if !defined(MBEDTLS_SSL_CONF_SET_TIMER) && \
+    !defined(MBEDTLS_SSL_CONF_GET_TIMER)
     mbedtls_ssl_set_timer_cb( &ssl, &timer, mbedtls_timing_set_delay,
                                             mbedtls_timing_get_delay );
+#else
+    mbedtls_ssl_set_timer_cb_ctx( &ssl, &timer );
+#endif
+#endif
 
     mbedtls_printf( " ok\n" );