nixos/device-tree: improve overlays support

Now allows applying external overlays either in form of
.dts file, literal dts context added to store or precompiled .dtbo.

If overlays are defined, kernel device-trees are compiled with '-@'
so the .dtb files contain symbols which we can reference in our
overlays.

Since `fdtoverlay` doesn't respect `/ compatible` by itself
we query compatible strings of both `dtb` and `dtbo(verlay)`
and apply only if latter is substring of the former.

Also adds support for filtering .dtb files (as there are now nearly 1k
dtbs).

Co-authored-by: georgewhewell <georgerw@gmail.com>
Co-authored-by: Kai Wohlfahrt <kai.wohlfahrt@gmail.com>

pkgs/os-specific/linux/device-tree/default.nix

@@ -1,16 +1,31 @@
 { stdenvNoCC, dtc, findutils }:
 
 with stdenvNoCC.lib; {
-  applyOverlays = (base: overlays: stdenvNoCC.mkDerivation {
+  applyOverlays = (base: overlays': stdenvNoCC.mkDerivation {
     name = "device-tree-overlays";
     nativeBuildInputs = [ dtc findutils ];
     buildCommand = let
-      quotedDtbos = concatMapStringsSep " " (o: "\"${toString o}\"") (toList overlays);
+      overlays = toList overlays';
     in ''
-      for dtb in $(find ${base} -name "*.dtb" ); do
-        outDtb=$out/$(realpath --relative-to "${base}" "$dtb")
-        mkdir -p "$(dirname "$outDtb")"
-        fdtoverlay -o "$outDtb" -i "$dtb" ${quotedDtbos};
+      mkdir -p $out
+      cd ${base}
+      find . -type f -name '*.dtb' -print0 \
+        | xargs -0 cp -v --no-preserve=mode --target-directory $out --parents
+
+      for dtb in $(find $out -type f -name '*.dtb'); do
+        dtbCompat="$( fdtget -t s $dtb / compatible )"
+
+        ${flip (concatMapStringsSep "\n") overlays (o: ''
+        overlayCompat="$( fdtget -t s ${o.dtboFile} / compatible )"
+        # overlayCompat in dtbCompat
+        if [[ "$dtbCompat" =~ "$overlayCompat" ]]; then
+          echo "Applying overlay ${o.name} to $( basename $dtb )"
+          mv $dtb{,.in}
+          fdtoverlay -o "$dtb" -i "$dtb.in" ${o.dtboFile};
+          rm $dtb.in
+        fi
+        '')}
+
       done
     '';
   });