binman: Read _multiple_data_files in the correct place

Move this to the ReadEntries() function where it belongs.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/etype/mkimage.py b/tools/binman/etype/mkimage.py
index e028c44..cb3e106 100644
--- a/tools/binman/etype/mkimage.py
+++ b/tools/binman/etype/mkimage.py
@@ -121,7 +121,6 @@
     """
     def __init__(self, section, etype, node):
         super().__init__(section, etype, node)
-        self._multiple_data_files = fdt_util.GetBool(self._node, 'multiple-data-files')
         self._mkimage_entries = OrderedDict()
         self._imagename = None
         self._filename = fdt_util.GetString(self._node, 'filename')
@@ -129,6 +128,8 @@
 
     def ReadNode(self):
         super().ReadNode()
+        self._multiple_data_files = fdt_util.GetBool(self._node,
+                                                     'multiple-data-files')
         self._args = fdt_util.GetArgs(self._node, 'args')
         self._data_to_imagename = fdt_util.GetBool(self._node,
                                                    'data-to-imagename')