If you've flashed a Production image from the Android Things Console onto your developer hardware or are otherwise unable to connect to your device using fastboot or adb, follow these steps:
Connect to the serial debug console of your board.
Connect power to the board. You'll see the following output from the console:
U-Boot 2017.03-00545-gb47d6e0 (May 03 2018 - 19:05:06 +0000) CPU: ... Reset cause: POR Board: ... ... Fastboot: Normal Hit any key to stop autoboot: 3
Hit Enter before the countdown completes to access the bootloader prompt. Type
fastboot usband hit Enter:
Fastboot: Normal Hit any key to stop autoboot: 0 => fastboot usb
The board is now in fastboot mode. Follow the instructions to Manually Flash Android Things.