It is desirable that the default config.boot of a new installation contain the current system component version string, thus avoiding calling all migration scripts on first boot. Simply adding this will fail to pick up certain system-specific configuration settings which are present in some migration scripts (example below). Without changing any existent migration scripts, we will add an activation step to vyos-router to provide any needed settings.
This will require a review of existing scripts to extract needed activation steps, but will offer authors of future migration scripts a clear distinction of what to add for migration (version syntax changes), respectively, activation (system settings).
https://github.com/vyos/vyos-1x/blob/current/src/migration-scripts/interfaces/20-to-21#L43-L50