Only the RHCK->UEK flavor cross needs grubby --set-default; a UEK7->UEK8
update stays in the kernel-uek lineage and auto-promotes on its own. Detect
the running kernel and act accordingly:
- UEK8: already on target, no-op.
- UEK7: populate the repo and install UEK8, then verify it auto-promoted
(warn with the manual grubby command if it did not) -- no grubby change.
- RHCK: install UEK8 and set the boot default explicitly, as before.
Also make an already-installed UEK8 skip the repo entirely so a disabled or
empty kernel repo can't block flipping the default, and correct the header
comment that claimed every transition needs grubby.
Mike Reeves
9a71f64a35