moby: package/ship the rtl8723cs bluetooth driver

with this, bluetooth works immediately after boot (well, maybe only
after the second boot after a deploy... logs are weird when NTP
activates).

machines/moby/default.nix

@@ -170,6 +170,7 @@
   #   anx7688-fw.bin  (USB-C -> HDMI bridge)
   #   ov5640_af.bin   (camera module)
   # hardware.firmware = [ config.mobile.device.firmware ];
+  hardware.firmware = [ pkgs.rtl8723cs-firmware ];
 
   system.stateVersion = "21.11";
 

pkgs/overlay.nix

@@ -12,6 +12,7 @@
     # not sure why i can't just do pkgs = next here
     pkgs = prev // { inherit ubootRaspberryPi4_64bit; };
   };
+  rtl8723cs-firmware = prev.callPackage ./rtl8723cs-firmware { };
 
   #### customized packages
   # nixos-unstable pleroma is too far out-of-date for our db

pkgs/rtl8723cs-firmware/default.nix

@@ -0,0 +1,36 @@
+{ lib, stdenv, fetchFromGitHub }:
+with lib;
+stdenv.mkDerivation {
+  pname = "rtl8723cs-firmware";
+  version = "2020-07-05";
+
+  src = fetchFromGitHub {
+    owner = "anarsoul";
+    repo = "rtl8723bt-firmware";
+    rev = "8840b1052b4ee426f348cb35e4994c5cafc5fbbd";
+    sha256 = "sha256-z6OZNDvGbU1g+U9aL/Pq6fB3l7Fxwq6EHSeHgrkqt78=";
+  };
+
+  dontBuild = true;
+
+  installPhase = ''
+    mkdir -p "$out/lib/firmware"
+    cp -R rtl_bt "$out/lib/firmware"
+  '';
+
+  meta = with lib; {
+    description = "Firmware for rtl8723bs and rtl8723cs";
+    # there are many sources for this, none of them authoritative.
+    # the original binaries likely come from some Realtek SDK, hardcoded into a C array
+    # if consistent with other drivers, but Realtek does not list this model in their
+    # downloads page.
+    # other sources:
+    # - <https://megous.com/git/linux-firmware>
+    # - <https://github.com/armbian/firmware>
+    homepage = "https://github.com/anarsoul/rtl8723bt-firmware";
+    license = licenses.unfreeRedistributableFirmware;
+    maintainers = with maintainers; [ colinsane ];
+    platforms = with platforms; linux;
+  };
+}
+