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/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";
+  };
+}