pylibfdt: Convert to Python 3

Build this swig module with Python 3.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/dtc/pylibfdt/libfdt.i_shipped b/scripts/dtc/pylibfdt/libfdt.i_shipped
index 76e61e9..53b70f8 100644
--- a/scripts/dtc/pylibfdt/libfdt.i_shipped
+++ b/scripts/dtc/pylibfdt/libfdt.i_shipped
@@ -624,7 +624,7 @@
         Raises:
             FdtException if no parent found or other error occurs
         """
-        val = val.encode('utf-8') + '\0'
+        val = val.encode('utf-8') + b'\0'
         return check_err(fdt_setprop(self._fdt, nodeoffset, prop_name,
                                      val, len(val)), quiet)