nixos/hardware.deviceTree: new module

Add support for custom device-tree files, and applying overlays to them.
This is useful for supporting non-discoverable hardware, such as sensors
attached to GPIO pins on a Raspberry Pi.

nixos/modules/system/activation/top-level.nix

@@ -46,8 +46,8 @@ let
 
         ln -s ${kernelPath} $out/kernel
         ln -s ${config.system.modulesTree} $out/kernel-modules
-        ${optionalString (pkgs.stdenv.hostPlatform.platform.kernelDTB or false) ''
-          ln -s ${config.boot.kernelPackages.kernel}/dtbs $out/dtbs
+        ${optionalString (config.hardware.deviceTree.package != null) ''
+          ln -s ${config.hardware.deviceTree.package} $out/dtbs
         ''}
 
         echo -n "$kernelParams" > $out/kernel-params

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.sh

@@ -75,9 +75,8 @@ addEntry() {
 
     copyToKernelsDir "$path/kernel"; kernel=$result
     copyToKernelsDir "$path/initrd"; initrd=$result
-    # XXX UGLY: maybe the system config should have a top-level "dtbs" entry?
-    dtbDir=$(readlink -m "$path/kernel/../dtbs")
-    if [ -d "$dtbDir" ]; then
+    dtbDir=$(readlink -m "$path/dtbs")
+    if [ -e "$dtbDir" ]; then
         copyToKernelsDir "$dtbDir"; dtbs=$result
     fi