rauc keyring requiring absolute path keyring

This is a post to remind myself why rauc extract fails with a strange error message: “Not enough substeps: check_bundle” as also described in this github issue. The error is really not useful and it comes up if the –keyring=path is not an absolute path. Even ~/<path> will trigger that error and message. As the cherry on the top it core dumps on exit.

This issue has since been fixed but Ubuntu 20.04 contains rauc 1.2 which has this issue.