Use overlay

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

overlays/runtime/merge/keyValue/default.nix

@@ -0,0 +1,11 @@
+final: prev:
+"${
+  prev.writeShellApplication {
+    name = "merge-key-value";
+    runtimeInputs = with final; [
+      coreutils
+      gawk
+    ];
+    text = builtins.readFile ./key-value.sh;
+  }
+}/bin/merge-key-value"

overlays/runtime/merge/keyValue/key-value.sh

@@ -0,0 +1,15 @@
+# shellcheck shell=bash
+
+source=$(realpath -m "$1")
+target=$(realpath -m "$2")
+
+if [[ -f "$target" ]]; then
+  temp=$(mktemp)
+  awk -F '=' 'NR==FNR{a[$1]=$0;next}($1 in a){$0=a[$1]}1' "$source" "$target" > "$temp"
+  mv "$temp" "$target"
+else
+  mkdir -p "$(dirname "$target")"
+  cp "$source" "$target"
+fi
+
+echo "Configuration file $target has been updated."