My Chromebook ARM dos not boot properly with 4.20.0-lpae either and reboot after 90s approx, same for 4.20.5-lpae. kernel 4.20.0-default works fine. I tried to disable EFI support which was added recently, but it did not change anything. I cannot confirm this is exactly the same bug as I have no serial output on this machine and screen stays black, but I think it is the same.