Update the cedit read/write functions to support menu items with
values.

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

At present menu items are stored according to their sequence number in
the menu. In some cases we may want to have holes in that sequence, or
not use a sequence at all.

Add a new 'value' property for menu items. This will be used for
reading and writing, if present. If there is no 'value' property, then
the normal sequence number will be used instead.

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

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>

Provide test coverage for the new expo object type, including building
and reading/writing settings.

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

Add a command to write cedit settings to CMOS RAM so that it can be
preserved across a reboot. This uses a simple bit-encoding, where each
field has a 'bit position' and a 'bit length' in the schema.

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

Tidy up this tool a little:

- define which arguments are needed
- split the enum values out into a header file
- warn if no enum values are found
- display the dtc error if something goes wrong
- avoid a Python traceback on error

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

Add some simple tests and a helpful script to make the configuration
editor easier to set up.

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