Parallella Community

Okay, solved. .dtb file was broken on BOOT partition.

The board is booting, to the Xilinx prompt and when it is looking for the Boot sector files, it cannot find them.
ERROR: Did not find a cmdline Flattened Device Tree
Could not find a valid device tree

You need to copy the relevant boot files for your board into the boot sector.
See how to do this in the quickstart guide

HDMI will only launch after the Linux boot