This writes to the MCU system-state buffer and updates the SoftwareClosed (formerly LegacyPoweroff) bit.

This is set by LgyBg during graceful reboot (power not *held*) to prompt a “Software closed” screen when the system returns to NATIVE_FIRM. If power was held during a LGY fatal error, this flag is not set.

This is also set when power is pressed in System Settings (as a reboot is enforced there), hence no longer being called “LegacyPoweroff”.