binman: Update Entry.ReadEntry() to work through classes

At present we simply extract the data directly from entries using the
image_pos information. This happens to work on current entry types, but
cannot work if the entry type encodes the data in some way. Update the
ReadData() method to provide the data by calling a new ReadChildData()
method in the parent. This allows the entry_Section class, or possibly
any other container class, to return the correct data in all cases.

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