flash_swagbadge.sh only generate keys when not found.

[simotek]

If configuration/keys.db has a non zero key then don't attempt to generate a new one. This means users can use mpfshell to read the key and network settings, add them to configuration/keys.db and configuration/net.py and use this script to update there 2021 badge.

I'm open to other approaches to pass through the key but this seamed the simplest.