After pacman-updating from 4.9.50-1 to 4.9.50-2 kernel on my XU4 w/eMMC, it fails to boot - no blue heartbeat LED.
I don't have the UART module to dig into the issue so I ended up removing the eMMC to mount it on my regular Linux desktop with the SD-eMMC adaptor. I was able to copy the /boot/dtbs/*.dtb files from the 4.9.50-1 kernel archive to replace the dtbs that came with the 4.9.50-2 release. After that, I was able to boot the XU4 again.
I did a full "pacman -U" manual downgrade on the kernel back to 4.9.50-1 for now.
Hopefully this saves somebody else a bit of grief.