This tool when run as root on the Amazon FireTV will unlock the bootloader and then reboot. It requires the bootloader from 51.1.0.2 or earlier.

git clone --recursive https://github.com/androidrbox/aftv-full-unlock.git

Ensure you have the android-ndk in your PATH.

make

adb push aftv-full-unlock /data/local/tmp
adb shell
$ su
# chmod 755 /data/local/tmp/aftv-full-unlock
# /data/local/tmp/aftv-full-unlock

After which is will unlock and reboot. After it restarts, running adb shell should result in a root prompt.

Thanks to everyone mentioned at http://forum.xda-developers.com/kindle-fire-hdx/orig-development/dev-bootloader-unlock-procedure-software-t3030281. I'm no quite sure who is responsible for what. I wrote most of this code months ago, before Cuber was released, but thanks to dpeddi and whoever else for figuring out the mmc ID stuff, I have decided to publish it.