From: Zbigniew Jędrzejewski-Szmek on gitlab.com
I think it'd make sense to make calls to kernel-install optional: i.e. do them
only if `/usr/bin/kernel-install` is present. The justification is that on
"normal" systems, systemd is installed and you want to call kernel-install.
But if the kernel is pulled in a container or a build root, you want to skip
the call, and right now we pull in systemd just to detect whether to skip.
Hmm, I wanted to do this at some later point, but if we're touching the
scriptlets here anyway, let's just do it here too.