add output for u-boot

modules/arch/aarch64.nix

@@ -13,5 +13,6 @@
       # USE_OF = "y";
     };
     hardware.ram.startAddress = lim.parseInt "0x40000000";
+    system.outputs.u-boot = pkgs.ubootQemuAarch64;
   };
 }

modules/arch/arm.nix

@@ -5,5 +5,6 @@
       OF = "y";
     };
     hardware.ram.startAddress = lim.parseInt "0x40000000";
+    system.outputs.u-boot = pkgs.ubootQemuArm;
   };
 }

modules/arch/mipseb.nix

@@ -5,5 +5,6 @@
     kernel.config = {
       CPU_BIG_ENDIAN = "y";
     };
+    system.outputs.u-boot = pkgs.ubootQemuMips;
   };
 }