initrd-vendor: init module

This is required for some devices, as some features only work when
firmwares are loaded on some devices.

Here, `google-walleye` `gsi.rndis` usb gadget requires the IPA firmware.

modules/initrd-vendor.nix

@@ -0,0 +1,37 @@
+# This module handles well-known "vendor" partitions.
+#
+# Relevant upstream documentation:
+#  * https://www.kernel.org/doc/html/v4.14/driver-api/firmware/fw_search_path.html
+#
+# Note that we'll be using `firmware_class.path=/vendor/firmware` on the
+# command-line to make the firmware path known ASAP without requiring run-time
+# configuration. The NixOS stage-2 will configure `/sys/module/firmware_class/parameters/path`
+# as expected.
+{ config, lib, ... }:
+
+let
+  device_info = config.mobile.device.info;
+  vendor_device =
+    if config.mobile.system.type == "android" then
+      if device_info ? ab_partitions && device_info.ab_partitions then
+        # Force slot "A" on A/B, we would recommend end-users to flash the most
+        # compatible vendor partition to both slots anyway.
+        "/dev/disk/by-partlabel/vendor_a"
+      else
+        # Assume "vendor" partlabel on other android devices.
+        "/dev/disk/by-partlabel/vendor"
+    else null
+  ;
+in
+lib.mkMerge [
+  (lib.mkIf (vendor_device != null) {
+    # FIXME: add "firmware_class.path=/vendor/firmware" to kernel cmdline
+    boot.specialFileSystems = {
+      "/vendor" = {
+        device = vendor_device;
+        fsType = "ext4";
+        options = [ "nosuid" "noexec" "nodev" ];
+      };
+    };
+  })
+]

modules/module-list.nix

@@ -24,6 +24,7 @@
   ./initrd-splash.nix
   ./initrd-ssh.nix
   ./initrd-usb.nix
+  ./initrd-vendor.nix
   ./initrd.nix
   ./mobile-device.nix
   ./nixpkgs.nix