base-files: sysupgrade: iopsys_upgrade: Explicitly return success or failure
Be more explicit in our exit value management in iopsys_upgrade. More or less:
- If we fail, explicitly return 1.
- If we succeed somewhere, chug along, until we either fail or we're at the end of the function, in which case we return 0.
Coupled with us relying on target-specific functions returning meaningful return values to us, this makes things a bit more robust for success or failure management for do_stage2.