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/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