Draft: base-files: do_stage2: Create iopsys_{pre|post}_do_upgrade functions for config migration
do_stage2 cleanups:
- create pre- and post-upgrade functions to be called by the platform
- move platform-called code to post-upgrade
- unify pre- and post-upgrade script caller functions (and make it called respectively at the right places)
This goes in tandem with a platform-specific MR for brcm63xx TBD.....