Add support for System76 hardware, specifically the Darter Pro 6.

This relies on new `system76`, `system76-acpi`, and `system76-io`
packages in `linuxPackages` in Nixpkgs.

system76/darp6/default.nix

@@ -0,0 +1,27 @@
+# Hardware profile for the Darter Pro 6 laptop by System76.
+#
+# https://system76.com/laptops/darter
+
+{ config, ... }:
+let
+  # darp6 needs system76-acpi-dkms, not system76-dkms:
+  #
+  # [1] https://github.com/pop-os/system76-dkms/issues/39
+  # jackpot51> system76-acpi-dkms is the correct driver to use on the darp6
+  #
+  # system76-io-dkms also appears to be loaded on darp6 with Pop!_OS, and
+  # system76-dkms does not, and in fact refuses to load.
+  packages = with config.boot.kernelPackages; [ system76-acpi system76-io ];
+in
+{
+  imports = [
+    ../../common/pc/laptop
+    ../../common/pc/laptop/ssd
+  ];
+
+  boot.extraModulePackages = packages;
+
+  # system76_acpi automatically loads on darp6, but system76_io does not.
+  # Explicitly list all modules to be loaded, for consistency.
+  boot.kernelModules = map (drv: drv.moduleName) packages;
+}

system76/default.nix

@@ -0,0 +1,22 @@
+# Implementation of support for general System76 hardware.
+#
+# https://system76.com/
+
+{ config, ... }:
+let
+  # Try loading all system76 modules.  The ones not relevant to specific
+  # hardware won't be loaded.
+  packages = with config.boot.kernelPackages; [ system76 system76-acpi system76-io ];
+in
+{
+  imports = [ ../common/pc ];
+
+  # This seems to be required for system76-driver.
+  boot.kernelParams = [ "ec_sys.write_support=1" ];
+
+  boot.extraModulePackages = packages;
+
+  # Explicitly attempt to load all available system76 modules.  Some do
+  # (system76-acpi), some don't (system76-io).
+  boot.kernelModules = map (drv: drv.moduleName) packages;
+}