Add option -K to allow customizing the keyboard layout

This MR adds the option to customize the keyboard layout of the VM image. This is useful for those of us that run multiple Kali VMs while not using the default us keyboard layout.

The scripts/generate-*.sh scripts had the hard coded string "us keyboard layout" as description, so the layout needs to be set for the rootfs build and image build itself. Please take a look at it, I am open for any kind of feedback.