turny: fix SPI/GPIO config for newer kernels

- Add complete config.txt settings with u-boot boot lines + spi=on
- Override sdImage.populateFirmwareCommands to use configtxt module output
- Mount firmware partition and add activation script to sync config.txt
- Load spi_bcm2835 and bcm2835_gpiomem kernel modules
- Add udev rule for gpiomem subsystem (not just gpio)
- Relax ProtectKernelTunables so service can access GPIO
- Update turny flake input to rppal-based CS pin fix

modules/temp-build-storage.nix

@@ -0,0 +1,38 @@
+{
+  lib,
+  config,
+  ...
+}:
+let
+  cfg = config.modules.tempBuildStorage;
+in
+{
+  options.modules.tempBuildStorage = {
+    enable = lib.mkEnableOption "temporary build storage volume";
+
+    device = lib.mkOption {
+      type = lib.types.str;
+      default = "/dev/sdb";
+      description = "Block device to mount for temporary nix build storage.";
+    };
+
+    mountPoint = lib.mkOption {
+      type = lib.types.str;
+      default = "/mnt/build";
+      description = "Mount point for the temporary build storage.";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    fileSystems.${cfg.mountPoint} = {
+      device = cfg.device;
+      fsType = "ext4";
+      options = [
+        "nofail"
+        "defaults"
+      ];
+    };
+
+    nix.settings.build-dir = cfg.mountPoint;
+  };
+}