Fix the boot-ec2-config test

nixos/modules/profiles/headless.nix

@@ -20,4 +20,7 @@ with lib;
 
   # Don't allow emergency mode, because we don't have a console.
   systemd.enableEmergencyMode = false;
+
+  # Being headless, we don't need a GRUB splash image.
+  boot.loader.grub.splashImage = null;
 }

nixos/tests/ec2.nix

@@ -20,6 +20,14 @@ let
               ln -s vda /dev/xvda
               ln -s vda1 /dev/xvda1
             '';
+
+          # Needed by nixos-rebuild due to the lack of network
+          # access. Mostly copied from
+          # modules/profiles/installation-device.nix.
+          system.extraDependencies =
+            [ pkgs.stdenv pkgs.busybox pkgs.perlPackages.ArchiveCpio
+              pkgs.unionfs-fuse pkgs.mkinitcpio-nfs-utils
+            ];
         }
       ];
     }).config.system.build.amazonImage;