Raspberry Pi bootup hangs with the ACT led (green light) flashing 8 times.
So what I did, I opened the micro SD card up on my laptop and searched for start.elf. I then deleted it and replaced it with a new start.elf found here: https://github.com/raspberrypi/firmware/blob/master/boot/start.elf.
Insert the card back into your Pi and it’ll bootup as normal.
This fixed the issue for Raspbian Stretch and Ubuntu MATE.