rpi-400: try to boot via EDK2

however, my build of edk2 seems broken. drop the prebuilt RPI_EFI.fd into the resulting image, and it works ...

hosts/modules/hal/rpi-400.nix

@@ -7,6 +7,13 @@ in
     sane.hal.rpi-400.enable = lib.mkEnableOption "Raspberry Pi 400 hardware support";
   };
   config = lib.mkIf cfg.enable {
-    sane.image.extraBootFiles = [ pkgs.bootpart-u-boot-rpi-aarch64 ];
+    sane.image.extraBootFiles = [
+      # rpi bootrom -> edk2 -> systemd-boot
+      # edk2 exists here to provide the base UEFI environment which systemd-boot expects
+      pkgs.bootpart-edk2-rpi
+      pkgs.bootpart-systemd-boot
+    ];
+    #v used by systemd-boot
+    hardware.deviceTree.name = "broadcom/bcm2711-rpi-400.dtb";
   };
 }