binman: Adjust Entry to read the node in a separate call

binman: Adjust Entry to read the node in a separate call

At present the Entry constructor sets up the object and then immediately
reads its device-tree node to obtain its properties.

This breaks a convention that constructors should not do any processing.
A consequence is that we must pass all arguments to the constructor and
cannot have the node-reading proceed in a different way unless we pass
flags to that constructor. We already have a 'test' flag in a few cases,
and now need to control whether the 'orig_offset' and 'orig_size'
properties are set or not.

Adjust the code to require a separate call to ReadNode() after
construction. The Image class remains as it was.

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