Fix ugly systemd-boot entries

nixos/systemd-boot.nix

@@ -1,5 +1,5 @@
 # Systemd-boot configuration for NixOS
-{ pkgs, ... }:
+{ pkgs, config, ... }:
 {
   boot = {
     bootspec.enable = true;
@@ -9,6 +9,22 @@
         enable = true;
         consoleMode = "auto";
         configurationLimit = 5;
+
+        extraInstallCommands = ''
+          ENTRIES="${config.boot.loader.efi.efiSysMountPoint}/loader/entries"
+          PROFILES="/nix/var/nix/profiles"
+
+          for file in "$ENTRIES"/nixos-generation-*.conf; do
+            generation=$(${pkgs.coreutils}/bin/basename "$file" | ${pkgs.gnugrep}/bin/grep -o -E '[0-9]+')
+            timestamp=$(${pkgs.coreutils}/bin/stat -c %y "$PROFILES/system-$generation-link" 2>/dev/null | ${pkgs.coreutils}/bin/cut -d. -f1)
+
+            if [ -z "$timestamp" ]; then
+              timestamp="Unknown Date"
+            fi
+
+            ${pkgs.gnused}/bin/sed -i "s/^version .*/version Generation $generation - $timestamp/" "$file"
+          done
+        '';
       };
     };
     tmp.cleanOnBoot = true;