sd-image add

hosts/harmonica/sd-image.nix

@@ -0,0 +1,40 @@
+# This module extends the official sd-image.nix with the following:
+# - ability to add options to the config.txt firmware
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}: {
+  options.sdImage = with lib; {
+    extraFirmwareConfig = mkOption {
+      type = types.attrs;
+      default = {};
+      description = lib.mdDoc ''
+        Extra configuration to be added to config.txt.
+      '';
+    };
+  };
+
+  config = {
+    sdImage.populateFirmwareCommands =
+      lib.mkIf ((lib.length (lib.attrValues config.sdImage.extraFirmwareConfig)) > 0)
+      (
+        let
+          # Convert the set into a string of lines of "key=value" pairs.
+          keyValueMap = name: value: name + "=" + toString value;
+          keyValueList = lib.mapAttrsToList keyValueMap config.sdImage.extraFirmwareConfig;
+          extraFirmwareConfigString = lib.concatStringsSep "\n" keyValueList;
+        in
+          lib.mkAfter
+          ''
+            config=firmware/config.txt
+            # The initial file has just been created without write permissions. Add them to be able to append the file.
+            chmod u+w $config
+            echo "\n# Extra configuration" >> $config
+            echo "${extraFirmwareConfigString}" >> $config
+            chmod u-w $config
+          ''
+      );
+  };
+}