rtl8821au: Fix build for ARM

pkgs/os-specific/linux/rtl8821au/default.nix

@@ -18,6 +18,15 @@ stdenv.mkDerivation rec {
 
   NIX_CFLAGS_COMPILE="-Wno-error=incompatible-pointer-types";
 
+  makeFlags = [
+    "ARCH=${stdenv.hostPlatform.linuxArch}"
+    "KSRC=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"
+    ("CONFIG_PLATFORM_I386_PC=" + (if stdenv.hostPlatform.isx86 then "y" else "n"))
+    ("CONFIG_PLATFORM_ARM_RPI=" + (if (stdenv.hostPlatform.isAarch32 || stdenv.hostPlatform.isAarch64) then "y" else "n"))
+  ] ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) [
+    "CROSS_COMPILE=${stdenv.cc.targetPrefix}"
+  ];
+
   prePatch = ''
     substituteInPlace ./Makefile \
       --replace /lib/modules/ "${kernel.dev}/lib/modules/" \
@@ -40,7 +49,7 @@ stdenv.mkDerivation rec {
     description = "rtl8821AU and rtl8812AU chipset driver with firmware";
     homepage = "https://github.com/morrownr/8821au";
     license = licenses.gpl2Only;
-    platforms = [ "x86_64-linux" "i686-linux" ];
+    platforms = lib.platforms.linux;
     maintainers = with maintainers; [ plchldr ];
   };
 }