It looks like whatever boot ROM the ESP32 has inside it is testing for a high voltage on GPIO 12. Don’t ask us. Anyway, the short version of the story is: pull GPIO 12 high and hit reset.
Got an nRF52 or nRF51 device you need to flash? Got an ESP32 laying around collecting dust? If so, then firmware hacking extraordinaire [Aaron Christophel] has the open source code you need.