expo: Use standard numbering for save and discard

expo: Use standard numbering for save and discard

Set aside some expo IDs for 'save' and 'discard' buttons. This avoids
needing to store the IDs for these. Adjust the documentation and expo
tool for the new EXPOID_BASE_ID value.

Ignore these objects when saving and loading the cedit, since they do
not contain real data.

Adjust 'cedit run' to return failure when the user exits the expo
without saving. Update the test for this change as well.

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