Update, got it to boot by... get this: plugging a device (in my case an audio interface) to the other USB port. For some reason, it won't boot without this.
Now the issue i'm having is that over half the devices aren't recognized by the kernel. The keyboard, screen, mousepad & USB ports seem to work fine but the rest doesn't: internal storage, wifi, bluetooth, (internal) sound chip, touch screen...
So i'm guessing i'll need to compile my own kernel to support these, assuming there is support in the mainline (or other) kernel source - this is what "DTB" (iirc device tree blob) files are supposed to assist in doing, right?