detect arch in kernel and uimage

also move kernel builder to pkgs/

FIXME we need to straighten out the mess in calling
dtb.nix/uimage.nix

pkgs/kernel/dtb.nix

@@ -0,0 +1,23 @@
+{
+  stdenv
+, dtc
+, lib
+}:
+{ dts
+, includes
+, commandLine
+}:let
+  cppDtSearchFlags = builtins.concatStringsSep " " (map (f: "-I${f}") includes);
+  dtcSearchFlags = builtins.concatStringsSep " " (map (f: "-i${f}") includes);
+  cmdline = lib.concatStringsSep " " commandLine;
+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 ${dts}
+    echo '/{ chosen { bootargs = ${builtins.toJSON cmdline}; }; };'  >> dtb.tmp
+    dtc ${dtcSearchFlags} -I dts  -O dtb -o $out dtb.tmp
+    test -e $out
+  '';
+}