uboot: (firmwareOdroidC2/C4) don't invoke patch tool, use patches = [] instead

https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/generic/setup.sh#L948
this can do it nicely.

Signed-off-by: Anton Arapov <anton@deadbeef.mx>

nixos/modules/system/boot/kexec.nix

@@ -0,0 +1,32 @@
+{ pkgs, lib, ... }:
+
+{
+  config = lib.mkIf (lib.meta.availableOn pkgs.stdenv.hostPlatform pkgs.kexec-tools) {
+    environment.systemPackages = [ pkgs.kexec-tools ];
+
+    systemd.services.prepare-kexec =
+      { description = "Preparation for kexec";
+        wantedBy = [ "kexec.target" ];
+        before = [ "systemd-kexec.service" ];
+        unitConfig.DefaultDependencies = false;
+        serviceConfig.Type = "oneshot";
+        path = [ pkgs.kexec-tools ];
+        script =
+          ''
+            # Don't load the current system profile if we already have a kernel loaded
+            if [[ 1 = "$(</sys/kernel/kexec_loaded)" ]] ; then
+              echo "kexec kernel has already been loaded, prepare-kexec skipped"
+              exit 0
+            fi
+
+            p=$(readlink -f /nix/var/nix/profiles/system)
+            if ! [[ -d $p ]]; then
+              echo "Could not find system profile for prepare-kexec"
+              exit 1
+            fi
+            echo "Loading NixOS system via kexec."
+            exec kexec --load $p/kernel --initrd=$p/initrd --append="$(cat $p/kernel-params) init=$p/init"
+          '';
+      };
+  };
+}