add per-device overlay

presently this is used to reference the appropriate kernel and (if
needed) openwrt source trees, but I would not be surprised if we find
other uses

kernel/default.nix

@@ -6,15 +6,9 @@
 
 , config
 , checkedConfig
+, sources
 }:
 let
-  source = fetchFromGitHub {
-    owner = "torvalds";
-    repo = "linux";
-    rev = "3d7cb6b04c3f3115719235cc6866b10326de34cd";  # v5.19
-    hash = "sha256-OVsIRScAnrPleW1vbczRAj5L/SGGht2+GnvZJClMUu4=";
-  };
-
   # The kernel is huge and takes a long time just to
   # download and unpack. This derivation creates
   # a source tree in a suitable shape to build from -
@@ -24,7 +18,7 @@ let
 
   tree = stdenvNoCC.mkDerivation {
     name = "spindled-kernel-tree";
-    src = source;
+    src = sources.kernel;
     phases = [ "unpackPhase" "patchPhase" "patchScripts" "installPhase" ];
     patches = [ ./random.patch ];
     patchScripts = ''
@@ -36,15 +30,6 @@ let
     '';
   };
 
-  openwrtSource = fetchFromGitHub {
-    name = "openwrt-source-tree";
-    repo = "openwrt";
-    owner = "openwrt";
-    rev = "a5265497a4f6da158e95d6a450cb2cb6dc085cab";
-    hash = "sha256-YYi4gkpLjbOK7bM2MGQjAyEBuXJ9JNXoz/JEmYf8xE8=";
-  };
-
-
 in rec {
   vmlinux = callPackage ./vmlinux.nix {
     inherit tree config checkedConfig;
@@ -53,7 +38,7 @@ in rec {
   uimage = callPackage ./uimage.nix { };
 
   dtb = callPackage ./dtb.nix {
-    openwrt = openwrtSource;
+    openwrt = sources.openwrt;
     kernel = tree;
   };
 }