Integrate meta-seconorth-distro/kirkstone-fix-overrides
Commit: seco-ne/yocto/layers/meta-seconorth-distro@30e18056
overrides: Fix _ vs : syntax, use mx6, mx6ull and mx8
We reintroduced the plain mx8, mx8mm and mx8mp overrides without -generic-bsp appendix. This changes the usage back to those.