boot: Use strlcpy() in label_boot()

This function is recommended instead of strncpy() since it always
terminates the string.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/pxe_utils.c b/boot/pxe_utils.c
index 82f217a..e969358 100644
--- a/boot/pxe_utils.c
+++ b/boot/pxe_utils.c
@@ -558,7 +558,7 @@
 		}
 
 		if (label->append)
-			strncpy(bootargs, label->append, sizeof(bootargs));
+			strlcpy(bootargs, label->append, sizeof(bootargs));
 
 		strcat(bootargs, ip_str);
 		strcat(bootargs, mac_str);