If the built-in keyboard of your laptop is short-circuited, for example, it is desirable that it is disabled even before the Display Manager, for it might cause trouble on the user/password input.
- You use the linux kernel.
- You use grub.
These instructions will help disabling the driver of the built-in keyboard directly on the kernel, via grub.
Usually, the built-in keyboard is managed by the i8042 module, which manages PS/2 keyboards and mice.