expo: Tidy up the expo.py tool and usage

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>

expo: Add tests for the configuration editor

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>

expo: Add tests for the configuration editor

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>