binman: Allow control of which entries to read

binman: Allow control of which entries to read

The ObtainContents() and GetEntryContents() methods in this file read
every single entry in the section. This is the common case.

However when one of the entries has had its data updated (e.g. with
'binman replace') we don't want to read it again from the file. Allow
the entry to be skipped, for this purpose. This is currently done in the
CBFS implementation, so adding it here will allow that to use more of
the entry_Section code.

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