Allow configuration of endpoint (cli/srv) at compile-time

Introduces MBEDTLS_SSL_CONF_ENDPOINT to allow to choose between
server- or client-builds at compile-time.

Impact on code-size:

|  | GCC 8.2.1 | ARMC5 5.06 | ARMC6 6.12 |
| --- | --- | --- | --- |
| `libmbedtls.a` (client only) before  | 18355 | 18815 | 21485 |
| `libmbedtls.a` (client only) after | 18219 | 18683 | 21347 |
| gain in Bytes (client only) | 136 | 132 | 138 |
| `libmbedtls.a` (server only) before  | 18715 | 18987 | 21883 |
| `libmbedtls.a` (server only) after | 18595 | 18823 | 21717 |
| gain in Bytes (server only) | 120 | 164 | 166 |
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 1d7c05d..86759e5 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1055,7 +1055,9 @@
      * Flags (bitfields)
      */
 
+#if !defined(MBEDTLS_SSL_CONF_ENDPOINT)
     unsigned int endpoint : 1;      /*!< 0: client, 1: server               */
+#endif /* !MBEDTLS_SSL_CONF_ENDPOINT */
     unsigned int transport : 1;     /*!< stream (TLS) or datagram (DTLS)    */
 #if !defined(MBEDTLS_SSL_CONF_AUTHMODE)
     unsigned int authmode : 2;      /*!< MBEDTLS_SSL_VERIFY_XXX             */
@@ -1381,13 +1383,18 @@
  */
 int mbedtls_ssl_session_reset( mbedtls_ssl_context *ssl );
 
+#if !defined(MBEDTLS_SSL_CONF_ENDPOINT)
 /**
  * \brief          Set the current endpoint type
  *
+ * \note           On constrained systems, this can also be configured
+ *                 at compile-time via MBEDTLS_SSL_CONF_ENDPOINT.
+ *
  * \param conf     SSL configuration
  * \param endpoint must be MBEDTLS_SSL_IS_CLIENT or MBEDTLS_SSL_IS_SERVER
  */
 void mbedtls_ssl_conf_endpoint( mbedtls_ssl_config *conf, int endpoint );
+#endif /* !MBEDTLS_SSL_CONF_ENDPOINT */
 
 /**
  * \brief           Set the transport type (TLS or DTLS).