PCH is not HDMI – PCH is most likely the headphone jack on the motherboard.
The issue of NUC12 (and beyond) HDMI support is known and is not a simple fix. It’s on the roadmap to fixing, but Intel has changed quite a bit about how this works, so it’s going to take some serious effort.
We only recently got samples of these boards in. I also have some NUC13’s on the way here from Intel, and I want to make sure my changes will be compatible with the future line too.
Anyway, NUC12 + HDMI will not work with ROCK until we add this support. If you need Linux + HDMI + NUC12 before then, use Ubuntu.