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/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 29d9906..496f7e1 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -683,6 +683,13 @@
 #define "MBEDTLS_SSL_CONF_SEND/RECV/RECV_TIMEOUT must be defined simultaneously"
 #endif
 
+#if ( defined(MBEDTLS_SSL_CONF_GET_TIMER) &&            \
+      !defined(MBEDTLS_SSL_CONF_SET_TIMER) ) || \
+    ( !defined(MBEDTLS_SSL_CONF_GET_TIMER) &&           \
+      defined(MBEDTLS_SSL_CONF_SET_TIMER) )
+#define "MBEDTLS_SSL_CONF_GET_TIMER and MBEDTLS_SSL_CONF_SET_TIMER must be defined together."
+#endif
+
 #if defined(MBEDTLS_SSL_TICKET_C) && !defined(MBEDTLS_CIPHER_C)
 #error "MBEDTLS_SSL_TICKET_C defined, but not all prerequisites"
 #endif