binman: Allow writing section contents to a file

At present only the image (which is a section) has a filename. Move this
implementation to the entry_Section class so that any section can have a
filename. With this, the section data is written to a file.

This allows parts of an image to be written, along with the entire image.

Make a note that this can be used to include the contents of a section in
one image in another (later) image.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/image.py

@@ -94,9 +94,6 @@ class Image(section.Entry_section):
 
     def ReadNode(self):
         super().ReadNode()
-        filename = fdt_util.GetString(self._node, 'filename')
-        if filename:
-            self._filename = filename
         self.allow_repack = fdt_util.GetBool(self._node, 'allow-repack')
         self._symlink = fdt_util.GetString(self._node, 'symlink')