[CoreOS] How to make ignition think it's the first boot?

[QingFeng Hao]

Hi,
According to the link https://coreos.com/ignition/docs/latest/boot-process.html, ignition
only takes effect at the first boot.
But we want to make it effect later if we missed something in the first boot, or we want
to debug ignition.
So question is: how can I set file coreos/first_boot in the EFI System Partition, or set
'coreos.first_boot=detected' in kernel parameter line in an existing
coreos qcow2 image which can be started, so that I can make ignition work?

Thanks a lot!

-- 
Regards
QingFeng Hao