So it seems that with the correct u-boot flashed, any latest generation Kindle devices can enter fastboot mode and download the Android ROM and run it. Am I right?
I found some general article regarding u-boot:
https://boundarydevices.com/u-boot-u...-core-2gb-ddr/
https://boundarydevices.com/fastboot-on-i-mx/
But I suppose the 'memory map' in u-boot mentioned by knc1 is device specific so it is crucial for rebuilding the u-boot image?
Anyway, these are just some of my wild guesses. My apology if it doesn't help.