How to diagnose the «Cron script readiness check failed» failure during a Magento 2 upgrade?

Administration Upgrade

dmitry_fedyuk (Dmitrii Fediuk) January 24, 2016, 8:04am 1

How is a «Cron script readiness check» implemented on a Magento 2 upgrade?
How to disable the «Cron script readiness check» on a Magento 2 upgrade?
How to setup and run cron for Magento 2?
Bug: Magento 2 checks the .update_cronjob_status file but never generates it so the «Cron script readiness check» will always fail on a Magento 2 upgrade (and my fix)