binman: Correct operation of ObtainContents()

binman: Correct operation of ObtainContents()

This method is supposed to return the contents of an entry. However at
present there is no check that it actually does. Also some implementations
do not return 'True' to indicate success, as required.

Add a check for things working as expected, and correct the
implementations.

This requires some additional test cases to cover things which were missed
originally. Add these at the same time.

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