Allow using different kernels

We currently hardcode the name of the kernel image.

This becomes problematic in the case where users would like to create a profile with a different kernel (e.g. linux-lts, linux-hardened, or custom) as the image name will be different.

I wonder whether it would be enough to install a the image to a specific location again (as was done before, and now I know why!), e.g.

install -m 0644 -- "${airootfs_dir}/boot/vmlinuz-"* "${isofs_dir}/${install_dir}/boot/${arch}/vmlinuz-linux"

However, the above does not catch a case in which users (accidentally) add multiple kernel packages (in such a case the above would not work and die on install: target 'XYZ' is not a directory).

I guess this could be remedied by applying some heuristics on the amount of images (and e.g. fail when there is more than one - as we can (currently) can not boot more than one anyways).

A user suggested an approach using a variable - this is still hardcoded though.