Yubikey PBA

Implement pre-boot authentication with a Yubikey for NixOS.
Features both two-factor authentication and using the Yubikey by itself (the former is recommended and the default)
The necessary setup to use this feature is shown here: https://gist.github.com/Calrama/9ed4d59f295f2431b651#file-setup_luks_device_for_pba-sh (Simply change the DEVICE, STORAGE, SLOT, YUBIKEYS, and MULTIUSER variables to your requirements; to not use two-factor authentication, simply press enter when asked for your user passphrase).