How to enter serial bootloader on the ESP32

In order to enter the serial bootloader on the ESP32, connect both GPIO0 and GPIO2 to GND while the ESP32 is starting up.

A typical hardware sequence to enter the bootloader is:

1. Connect GPIO0 and GPIO2 to GND
2. Connect the EN or RST pin to GND for at least 100ms, then connect to 3.3V

The second step will restart the ESP32.