Does the nightly image boot?
I don't have a git branch named gbm-bt2020 and I know nothing about archlinux.
Sorry I got confused, I'm building kodi from https://github.com/lrusak/xbmc - I figured from reading here that this was the sources you where using - mind telling me which commit you're using? You seem to be using git sha 3d12daa7f7a71d89c4740193948c64e341fa5b73but I'm not sure where this is from
Turns out i'm an idiot, seems librelec has a well known bug on NUC7 where F10 boot doesnt work but boot from F2 does. Image boots and HDR is working well with my LG C2.