Collapse the Gentoo kernel scripts into one, and move the install into a gentoo set

[RWejlgaard]

scripts/gentoo-kernel-upgrade-openrc and scripts/gentoo-kernel-upgrade-systemd were ~400 lines each and differed on three lines — whether the kernel image in /boot is named vmlinuz-<version> or kernel-<version>. Everything else was duplicated verbatim, so any fix had to be made twice.

Collapsing them

One scripts/gentoo-kernel-upgrade that detects the naming at runtime, cheapest reliable signal first:

1. Whichever name the running kernel is installed under — that's the name this system's installkernel actually used.
2. Failing that, whatever else is in /boot (the running kernel may already have been cleaned up).
3. Failing that, the init system, which is what picked the layout to begin with.

KERNEL_PREFIX=kernel overrides it if the guess is ever wrong.

That also kills the interactive "which init system? [1]/2" prompt in 07-last-touches.sh, which blocked unattended installs on Gentoo.

The gentoo set

Installing that helper was the one thing 07-last-touches.sh did that isn't really a last touch — it fired on every Gentoo box whether the script was wanted or not. It's config/sets/gentoo/ now, opt-in via make picky like the desktop sets.

os can only be as specific as uname -s, so the set is offered on any Linux and checks /etc/gentoo-release itself at deploy time, warning and exiting 0 elsewhere — the same shape as gnome/kde/xfce checking for their own tooling.

Also here

• config/sets/*/apply.sh added to the shellcheck glob. Without it the new script would ship unlinted. It immediately caught an unquoted expansion in the kernel-upgrade helper; the other four apply.sh scripts were already clean.
• README: gnome documented. It never got an entry when it landed — missing from both the layout tree and the set list.

Testing

• Full CI shellcheck glob clean; bash -n under /bin/bash 3.2 on macOS.
• Prefix detection: 8 cases (explicit override, running kernel under each name, both names present with each as the running one, only-superseded images under each name, empty /boot) — all pass, under bash 5 and bash 3.2.
• End-to-end on a real gentoo/stage3 image: set deploys, helper lands in /usr/bin, runs and reports the naming it detected.
• On ubuntu:24.04: gentoo shows in the picker, skips with a warning off-Gentoo, and basic still deploys alongside it.
• docker buildx build -f tests/Dockerfile-alpine . green.

Not fixed, noticed while in here

cleanup_old_versions() reads $current_version out of main's scope via bash's dynamic scoping rather than taking it as an argument. It works, and it predates this change, so I left it alone.