nixos/hardware.deviceTree: Move things around

In response to comments, create a sub-folder for deviceTree packages
(starting with rpi), and a top-level package for helpers.

pkgs/os-specific/linux/device-tree/default.nix

@@ -0,0 +1,17 @@
+{ stdenvNoCC, dtc, findutils }:
+
+with stdenvNoCC.lib; {
+  applyOverlays = (base: overlays: stdenvNoCC.mkDerivation {
+    name = "device-tree-overlays";
+    nativeBuildInputs = [ dtc findutils ];
+    buildCommand = let
+      quotedDtbos = concatMapStringsSep " " (o: "\"${toString o}\"") (toList overlays);
+    in ''
+      for dtb in $(find ${base} -name "*.dtb" ); do
+        outDtb=$out/$(realpath --relative-to "${base}" "$dtb")
+        mkdir -p "$(dirname "$outDtb")"
+        fdtoverlay -o "$outDtb" -i "$dtb" ${quotedDtbos};
+      done
+    '';
+  });
+}

pkgs/os-specific/linux/device-tree/raspberrypi.nix

@@ -0,0 +1,14 @@
+{ stdenvNoCC, raspberrypifw }:
+
+stdenvNoCC.mkDerivation {
+  name = "raspberrypi-dtbs-${raspberrypifw.version}";
+  nativeBuildInputs = [ raspberrypifw ];
+  buildCommand = ''
+    mkdir -p $out/broadcom/
+    cp ${raspberrypifw}/share/raspberrypi/boot/bcm*.dtb $out/broadcom
+  '';
+  passthru = {
+    # Compatible overlays that may be used
+    overlays = "${raspberrypifw}/share/raspberrypi/boot/overlays";
+  };
+}