kernel-install refactorings (#40610)

This contains the first two commits from #38764. While @daandemeyer
convinced me to base systemd-sysinstall on a new "bootctl link" rather
than "kernel-install", I think the refactorings I prepped as part of the
original work still make a lot of sense on their own, and I hope I
didn't do them for /dev/null.