Following up https://github.com/NixOS/nixpkgs/pull/23665
Bootable USB-drives are not limited to ISO-images, there can be "normal" MBR/GPT-partitioned disk connected via USB-rack.
Also, "uas" implies "usb-storage", so there is no need to mention both.
* the keyboard modules in all-hardware.nix are already defaults of
boot.initrd.availableKernelModules
* ide modules, hid_lenovo_tpkbd and scsi_wait_scan have been removed
because they're not available anymore
* i8042 was a duplicate (see few lines abowe)
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.
