Due the recent inclusion of broadcom-bt-firmware in enableAllFirmware,
it was required to set `nixpkgs.config.allowUnfree` to obtain the full
list. To make this dependency more explicit an assertion is added and an
alternative option `enableRedistributalFirmware` is provided to only
obtain firmware with an license allowing redistribution.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
