make config.hardware.dts.src nullable

This is for QEMU where we won't have to provide a dtb because the
device tree is built by the platform according the (emulated) hardware
present.

Maybe in future there will be other hardware devices where we
don't need to provide a dtb.

modules/hardware.nix

@@ -16,8 +16,14 @@ in {
     hardware = {
       dts = {
         src = mkOption {
-          type = types.path;
-          description = "Path to the device tree source (usually from OpenWrt)";
+          type = types.nullOr types.path;
+          description = ''
+            If the device requires an external device tree to be loaded
+            alongside the kernel, this is the path to the device tree source
+            (we usually get these from OpenWrt). This value may be null if the
+            platform creates the device tree - currently this is the case
+            only for QEMU.
+          '';
         };
         includes = mkOption {
           default = [];

modules/outputs/extlinux.nix

@@ -9,6 +9,7 @@ let
   cfg = config.boot.loader.extlinux;
   o = config.system.outputs;
   cmdline = concatStringsSep " " config.boot.commandLine;
+  wantsDtb = config.hardware.dts ? src && config.hardware.dts.src != null;
 in {
   options.system.outputs.extlinux = mkOption {
     type = types.package;
@@ -20,18 +21,18 @@ in {
     system.outputs.extlinux = pkgs.runCommand "extlinux" {} ''
       mkdir $out
       cd $out
-      # cp {o.dtb} dtb
+      ${if wantsDtb then "cp ${o.dtb} dtb" else "true"}
       cp ${o.initramfs} initramfs
-      gzip -9f < ${o.kernel} > kernel.gz
+      cp ${o.zimage}  kernel
       mkdir extlinux
       cat > extlinux/extlinux.conf << _EOF
       menu title Liminix
       timeout 100
       label Liminix
-        kernel /boot/kernel.gz
-        initrd /boot/initramfs
-        append ${cmdline}
-        # fdt /boot/dtb
+        kernel /boot/kernel
+        # initrd /boot/initramfs
+        append ${cmdline} root=/dev/vda1
+        ${if wantsDtb then "fdt /boot/dtb" else ""}
       _EOF
     '';
   };