Allow compile-time configuration of authentication mode

Introduces MBEDTLS_SSL_CONF_AUTHMODE to fix the authentication
mode (none, optional, mandatory) at compile-time.

Impact on code-size:

|  | GCC | ARMC5 | ARMC6 |
| --- | --- | --- | --- |
| `libmbedtls.a` before | 23487 | 24025 | 27885 |
| `libmbedtls.a` after  | 23379 | 23929 | 27727 |
| gain in Bytes | 108 | 96 | 157 |
diff --git a/programs/ssl/query_config.c b/programs/ssl/query_config.c
index 2a7ca13..2557675 100644
--- a/programs/ssl/query_config.c
+++ b/programs/ssl/query_config.c
@@ -2578,6 +2578,14 @@
     }
 #endif /* MBEDTLS_PLATFORM_GMTIME_R_ALT */
 
+#if defined(MBEDTLS_SSL_CONF_AUTHMODE)
+    if( strcmp( "MBEDTLS_SSL_CONF_AUTHMODE", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_SSL_CONF_AUTHMODE );
+        return( 0 );
+    }
+#endif /* MBEDTLS_SSL_CONF_AUTHMODE */
+
 #if defined(MBEDTLS_SSL_CONF_ANTI_REPLAY)
     if( strcmp( "MBEDTLS_SSL_CONF_ANTI_REPLAY", config ) == 0 )
     {