Makefile.inc1: Fix INSTALLEXTRAKERNELS for one-kernel case

[ivy]

Summary

If BUILDKERNELS contains a single kernel, and NO_INSTALLEXTRAKERNELS=no,
then ${BUILDKERNELS:[2..-1]} returns the first kernel in the list and
that kernel ends up in both INSTALLKERNEL and INSTALLEXTRAKERNELS.

This breaks the package build because it means we generate that package
twice, once with an incorrect install path of /boot/kernel.NAME instead
of /boot/kernel.

Only set INSTALLEXTRAKERNELS if we're building at least two kernels.

Fixes: 2c0e9a76c1ce ("Makefile.inc1: Make NO_INSTALLKERNEL less of a hack")

[jrtc27]

That's... unhelpful. Also not sure how I managed to miss testing that case :(

Seems like this is a confusing result of "If start is greater than end, the words are output in reverse order" - -1 becomes 1 as there's only 1 item in the list, and that's before 2 (I note that "greater" is a confusing term in the manpage, too, given you can index from the end using negative integers). Of course, reversing the one-element list does nothing, but playing around with other similar ranges I can see it reversing them.

[ivy]

use .elif to make this a bit neater

[ivy]

In D54539#1246074, @jrtc27 wrote:

Seems like this is a confusing result of "If start is greater than end, the words are output in reverse order"

yes, i was rather surprised by this and thought it was a make bug at first, but it's actually documented behaviour... i feel like there should be a syntax for what we actually want to do here, but i couldn't find one.