binman: Support writing symbols inside a mkimage image

Add support for writing symbols and determining the assumed position of
binaries inside a mkimage image. This is useful as an example for other
entry types which might want to do the same thing.

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

tools/binman/entry.py

@@ -1314,8 +1314,6 @@ features to produce new behaviours.
         """
         data = b''
         for entry in entries:
-            # First get the input data and put it in a file
-            entry.ObtainContents(fake_size=fake_size)
             data += entry.GetData()
         uniq = self.GetUniqueName()
         fname = tools.get_output_filename(f'{prefix}.{uniq}')