notice: Wii U linux is not production ready! Proceed at your own risk. Written on 2/3/18
Quarktheawesome and rw-r-r_0644 have released an early version of Linux for Wii U!
The PowerPC side of the Wii U requires a custom linux kernel, which can be found here. The ARM side of the Wii U requires a custom fw.img file to load that kernel, which can be found here.
Once the kernel is up and running, the first repo is configured to automatically try to load programs from /dev/sda1 in an inserted usb. The first partition of an ext4 USB stick should contain a root filesystem for a linux distribution. See here for more information on a prebuilt PowerPC image