Just after some advice, on newer kernels provided from core the system would not boot. The system hangs on "Starting Kernel..." from serial tty.
Currently stuck on 4.5.4-1-ARCH as the last working version for my armv7h arch on ReadyNAS 104. I appreciate the system is unsupported - but I've expended all my technical knowledge on trying to get it working.
The GPL source for ReadyNAS OS 6.6.0 has uboot source inside, but will not compile due to missing source files. The version on the readynas is some build from 2013. Pacman reports that a uboot update is required, but without the GPL source compiling I dont know where to start with trying to compile the uboot source from denx.de. The denx.de source has no files for mv_ebu boards.