binman: Allow entry args to be required

binman: Allow entry args to be required

If an entry argument is needed by an entry but the entry argument is not
present, then a strange error can occur when trying to read the file.

Fix this by allowing arguments to be required. Select this option for the
cros-ec-rw entry. If a filename is provided in the node, allow that to be
used.

Also tidy up a few related tests to make the error string easier to find,
and fully ignore unused return values.

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