Add MBEDTLS_SSL_CONF_TRANSPORT

Follow the model of `MBEDTLS_SSL_CONF_ENDPOINT`. This saves a small
amount - most of the saving was already acheived via`
MBEDTLS_SSL_TRANSPORT_IS_TLS` but we can scrape out a little more by
totally eliminating `ssl->conf->transport` references.

Signed-off-by: Kevin Bracey <kevin.bracey@arm.com>
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 17c5763..ea60ef3 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -1454,6 +1454,21 @@
 }
 #endif /* MBEDTLS_SSL_CONF_ENDPOINT */
 
+#if !defined(MBEDTLS_SSL_CONF_TRANSPORT)
+static inline unsigned int mbedtls_ssl_conf_get_transport(
+    mbedtls_ssl_config const *conf )
+{
+    return( conf->transport );
+}
+#else /* !MBEDTLS_SSL_CONF_TRANSPORT */
+static inline unsigned int mbedtls_ssl_conf_get_transport(
+    mbedtls_ssl_config const *conf )
+{
+    ((void) conf);
+    return( MBEDTLS_SSL_CONF_TRANSPORT );
+}
+#endif /* MBEDTLS_SSL_CONF_TRANSPORT */
+
 #if !defined(MBEDTLS_SSL_CONF_READ_TIMEOUT)
 static inline uint32_t mbedtls_ssl_conf_get_read_timeout(
     mbedtls_ssl_config  const *conf )