Migrate old keyring to new box2 keyring

[Powersource]

Followup from here #96 (comment)

Probably easiest to add an opt to box2 to be able to set the full path, then we can just keep using the old dir

• Add config.box2.path option ssb-box2#36
• Keep using keyring storage from older versions #108

[Powersource]

old code said

// TODO: ssb-box2 stores the keyring at config.path/keyring. we're gonna have to merge this right?
  KeyRing(join(config.path, 'tribes/keystore'), (err, api) => {
    if (err) throw err