Set kernel-parameters for build-iso

[frelon]

The build-iso command sets hardcoded kernel parameters here https://github.com/rancher/elemental-cli/blob/133e72f7b26d6d2b0663545861b7202d000d0930/pkg/live/common.go#LL66C53-L66C53

It should be possible to influence those parameters by cmdline flags or config-file.

This issue is probably not a duplicate of #1773

[frelon]

After #1766 is merged I'm starting to think we can move these embedded config files into a live-grub-config feature.