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>
