[Skiboot] Minor FWTS integration improvements

[Jack Miller]

All -

I know Stewart's wrapping things up for 5.4.0 so I don't expect this to get
merged any time soon, but I've written a couple of enhancements for FWTS
integration and would appreciate any feedback.

The first patch adds a simple merge script that will take two olog JSON files
generated by the existing generate-fwts-olog script and output one that
includes error definitions from both versions. It will detect if the pattern
changed for a specific error and prompt the user to either duplicate that
error definition with both patterns or define a pattern that will cover both.

Right now, this script is extremely simple and takes no optional arguments.
The only enhancement I can think of is maybe a -f flag to go non-interactive,
but as it stands I think we're actually interested in changing definitions
more than automating this script.

The second patch is a tweak to generate-fwts-olog that uses subprocess to
invoke `git describe` and add a 'last-tag' field to the generated JSON. This
will make it easier to clean ancient error definitions out of merged ologs in
the future.

Again, fairly simple change, but perhaps could use a bit of polish. For
example, if anyone cares about running the script on non-git directories,
perhaps it could use an option / environment variable to manually set
last-tag, but that could just be needless complication.

Any suggestions welcome.

- Jack