raspberrypi-bootloader: Adds support for using uboot

Uboot is copied into `/boot` in the installation process.
The boot entries are created by calling the `generic-extlinux-compatible` builder.

nixos/modules/system/boot/loader/raspberrypi/raspberrypi.nix

@@ -5,42 +5,99 @@ with lib;
 let
   cfg = config.boot.loader.raspberryPi;
 
-  builder = pkgs.substituteAll {
+  builderGeneric = pkgs.substituteAll {
     src = ./builder.sh;
     isExecutable = true;
     inherit (pkgs) bash;
     path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
     firmware = pkgs.raspberrypifw;
     version = cfg.version;
+    inherit configTxt;
   };
 
   platform = pkgs.stdenv.platform;
 
+  builderUboot = import ./builder_uboot.nix { inherit config; inherit pkgs; inherit configTxt; };
+
+  builder = 
+    if cfg.uboot.enable then
+      "${builderUboot} -g ${toString cfg.uboot.configurationLimit} -t ${timeoutStr} -c"
+    else
+      builderGeneric;
+
+  blCfg = config.boot.loader;
+  timeoutStr = if blCfg.timeout == null then "-1" else toString blCfg.timeout;
+
+  isAarch64 = pkgs.stdenv.isAarch64;
+  optional = pkgs.stdenv.lib.optionalString;
+
+  configTxt =
+    pkgs.writeText "config.txt" (''
+      # U-Boot used to need this to work, regardless of whether UART is actually used or not.
+      # TODO: check when/if this can be removed.
+      enable_uart=1
+
+      # Prevent the firmware from smashing the framebuffer setup done by the mainline kernel
+      # when attempting to show low-voltage or overtemperature warnings.
+      avoid_warnings=1
+    '' + optional isAarch64 ''
+      # Boot in 64-bit mode.
+      arm_control=0x200
+    '' + optional cfg.uboot.enable ''
+      kernel=u-boot-rpi.bin
+    '');
+
 in
 
 {
   options = {
 
-    boot.loader.raspberryPi.enable = mkOption {
-      default = false;
-      type = types.bool;
-      description = ''
-        Whether to create files with the system generations in
-        <literal>/boot</literal>.
-        <literal>/boot/old</literal> will hold files from old generations.
-      '';
-    };
+    boot.loader.raspberryPi = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          Whether to create files with the system generations in
+          <literal>/boot</literal>.
+          <literal>/boot/old</literal> will hold files from old generations.
+        '';
+      };
 
-    boot.loader.raspberryPi.version = mkOption {
-      default = 2;
-      type = types.enum [ 1 2 3 ];
-      description = ''
-      '';
-    };
+      version = mkOption {
+        default = 2;
+        type = types.enum [ 1 2 3 ];
+        description = ''
+        '';
+      };
 
+      uboot = {
+        enable = mkOption {
+          default = false;
+          type = types.bool;
+          description = ''
+            Enable using uboot as bootmanager for the raspberry pi.
+          '';
+        };
+
+        configurationLimit = mkOption {
+          default = 20;
+          example = 10;
+          type = types.int;
+          description = ''
+            Maximum number of configurations in the boot menu.
+          '';
+        };
+
+      };
+    };
   };
 
-  config = mkIf config.boot.loader.raspberryPi.enable {
+  config = mkIf cfg.enable {
+    assertions = singleton {
+      assertion = !pkgs.stdenv.isAarch64 || cfg.version == 3;
+      message = "Only Raspberry Pi 3 supports aarch64.";
+    };
+
     system.build.installBootLoader = builder;
     system.boot.loader.id = "raspberrypi";
     system.boot.loader.kernelFile = platform.kernelTarget;