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/library/ssl_srv.c b/library/ssl_srv.c
index 0009f8b..b6b7750 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -55,7 +55,7 @@
                                  const unsigned char *info,
                                  size_t ilen )
 {
-    if( ssl->conf->endpoint != MBEDTLS_SSL_IS_SERVER )
+    if( mbedtls_ssl_conf_get_endpoint( ssl->conf ) != MBEDTLS_SSL_IS_SERVER )
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
 
     mbedtls_free( ssl->cli_id );