pkgs: add tow-boot-rpi4

NB: haven't tested the build

pkgs/overlay.nix

@@ -2,6 +2,7 @@
   #### my own, non-upstreamable packages:
   sane-scripts = prev.callPackage ./sane-scripts { };
   tow-boot-pinephone = prev.callPackage ./tow-boot-pinephone { };
+  tow-boot-rpi4 = prev.callPackage ./tow-boot-rpi4 { };
 
 
   #### customized packages

pkgs/tow-boot-rpi4/default.nix

@@ -0,0 +1,30 @@
+{ lib, stdenv, fetchurl }:
+
+stdenv.mkDerivation rec {
+  pname = "tow-boot-rpi4";
+  version = "2021.10-004";
+
+  src = fetchurl {
+    url = "https://github.com/Tow-Boot/Tow-Boot/releases/download/release-2021.10-004/raspberryPi-aarch64-2021.10-004.tar.xz";
+    sha256 = "sha256-UZSzzzTp9PQ/wuLUA3RJyTa/vbQ0HdhfagJ8574leoA=";
+  };
+
+  unpackPhase = ''
+    mkdir -p src
+    tar -xf ${src} -C src
+  '';
+
+  installPhase = ''
+    mkdir -p "$out"
+    cp -R src/raspberryPi-aarch64-2021.10-004/*.img "$out"/
+    cp -R src/raspberryPi-aarch64-2021.10-004/binaries/* "$out"/
+  '';
+
+
+  meta = with lib; {
+    description = "An opinionated distribution of U-Boot";
+    homepage = "https://tow-boot.org/";
+    platforms = [ "aarch64-linux" ];
+  };
+}
+