Trim down `pkgs.linux-firmware` to fit actually used hardware

[yfyf]

Pretty much all the firmware lives in a single package linux-firmware, which contains support for very diverse hardware and is getting bloated with each NixOS release (see NixOS/nixpkgs#148197), without an easy way to trim it down.

Since we mostly target specific x86 hardware with Intel CPUs using UHD, we can remove a lot of firmware for the unused hardware (AMD, Nvidia, legacy, etc).

PoC override that does this:

self: super: {
  linux-firmware = super.linux-firmware.overrideAttrs (old: {
    postInstall = (old.postInstall or "") + ''
      cd $out/lib/firmware

      # ARM SoC / embedded platforms
      rm -rf qcom mrvl airoha amlogic amphion arm cadence cnm dpaa2 \
             imx meson microchip nxp powervr rockchip ti-keystone \
             ti-connectivity inside-secure cypress

      # Atheros (Qualcomm) WiFi
      rm -rf ath6k ath9k_htc ath10k ath11k ath12k

      # NVIDIA GPU firmware
      rm -rf nvidia noveau

      # Mellanox / ConnectX server NICs
      rm -rf mellanox

      # Delete broken symlinks
      find $out -xtype l -delete
    '';
  });
}

The difference on nixpkgs 25.11 is more than 2x (~1GB smaller):

• Size of pkgs.linux-firmware as used in PlayOS "as is": 1744M
• Size of pkgs.linux-firmware as used in PlayOS with the overlay: 793M

The absolute size difference when compressed is smaller, but it still seems to yield a ~500M reduction in the compressed RAUC bundle size.

[yfyf]

The tricky question is deciding on what is safe to drop and how specialized we want the image to be.

Some data points:

• ArchLinux by default excludes various enterprise and SoC firmware, so this is a confirmation that we can safely drop it. They still ship most of the firmware by default.
• Debian runs hw-detect during installation to determine what firmware to pull in based on the current hardware booted.
• Fedora detects the hardware by scanning the hardware (via modalias strings) during regular package updates in the package manager (dnf) and updates the firmware "dynamically"

We can take several approaches:

• Gentle trim: drop only esoteric firmware we are sure not to need ever. Will ensure PlayOS images still run on most generic hardware. This is conservative, matches what ArchLinux does.
• Vendor trim: drop above + firmware from hardware vendors our builds do not currently use (Nvidia, AMD, Atheros, etc). Will make it impossible to use PlayOS images on some of the generic hardware.
• Max trim: aim to identify the exact hardware used and firmware needed for it (similar to hw-detect) on current PCs and include only that. Biggest reduction, but the most risky, since there's various "variations" of hardware on which PlayOS runs, so we could break older installations.

I think the optimal thing for us is to Vendor trim + verify on several generations of existing PCs what firmware they actually load/use. We could stick to Gentle trim in the portable flavor as a convenience.