build dtb (forgot to commit this file?)

kernel/dtb.nix

@@ -0,0 +1,24 @@
+{
+  stdenv
+, openwrt
+, dtc
+, kernel
+}:
+{ dts
+}:let
+  includes = [
+    "${openwrt}/target/linux/ath79/dts"
+    "${kernel}/include"
+  ];
+  cppDtSearchFlags = builtins.concatStringsSep " " (map (f: "-I${f}") includes);
+  dtcSearchFlags = builtins.concatStringsSep " " (map (f: "-i${f}") includes);
+in stdenv.mkDerivation {
+  name = "dtb";
+  phases = [ "buildPhase" ];
+  nativeBuildInputs = [ dtc ];
+  buildPhase = ''
+    ${stdenv.cc.targetPrefix}cpp -nostdinc -x assembler-with-cpp ${cppDtSearchFlags} -undef -D__DTS__  -o dtb.tmp ${openwrt}/target/linux/ath79/dts/${dts}
+    dtc ${dtcSearchFlags} -I dts  -O dtb -o $out dtb.tmp
+    test -e $out
+  '';
+}