binman: Tidy up setting of entry contents

binman: Tidy up setting of entry contents

At present the contents of an entry are set in subclasses simply by
assigning to the data and content_size properties. Add some methods to do
this, so that we have more control. In particular, add a method to set the
contents without changing its size, so we can validate that case.

Add a test case for trying to change the size when this is not allowed.

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