Add server certificate verfiy

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index c9de3b4..2974057 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -1448,7 +1448,6 @@
  */
 int ssl_tls13_write_server_certificate( mbedtls_ssl_context *ssl )
 {
-    ((void) ssl);
     int ret = mbedtls_ssl_tls13_write_certificate( ssl );
     if(ret != 0)
         return( ret );
@@ -1462,8 +1461,12 @@
  */
 int ssl_tls13_write_certificate_verify( mbedtls_ssl_context *ssl )
 {
-    ((void) ssl);
-    return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
+    int ret = mbedtls_ssl_tls13_write_certificate_verify( ssl );
+    if(ret != 0)
+        return( ret );
+
+    mbedtls_ssl_handshake_set_state( ssl, MBEDTLS_SSL_SERVER_FINISHED );
+    return( 0 );
 }
 
 /*