The README branch-selection criterion ("use legacy-mcu1 when the log
reports chip_id=7, chip_mcu_id=1 ...") is only observable on legacy-mcu1:
that branch's system_config_8800dc() logs chip_mcu_id, main's copy does
not, so DC devices running this branch can never see the criterion and
cannot distinguish mcu_id=0 from "unknown" (whose README fallback is
"keep using main"). Mirror the legacy format so the criterion is visible
where it is needed. One line; the value is computed two statements earlier.
