rootfs-handoff: implement rootwait boot parameter

rootfs-handoff: implement rootwait boot parameter

- Implemented rootwait to wait indefinitely for root device to appear
- Uses 30-second timeout to avoid infinite wait
- Waits 1 second between checks using process_sleep()
- Logs when device appears or timeout occurs
- Removed placeholder message for rootwait
- This completes the 'rootwait' item from ROOTFS_HANDOFF_PLAN.md Milestone E

Test Results:
- Smoke test: 131/131 PASS
- Battery test: 157/157 PASS
- Analyzer: PASS
- Zero regressions