nixos/installer/cd-dvd/iso-image: Honor boot.loader.timeout for EFI

EFI boot timeout is currently a static 10 whereas syslinux uses
boot.loader.timeout. This changes the EFI config to match.

Some discussion at https://discourse.nixos.org/t/how-to-override-let-variables/23741/2

(cherry picked from commit 7e1a3e4a8f)

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -79,6 +79,14 @@ let
 
   max = x: y: if x > y then x else y;
 
+  # Timeout in grub is in seconds.
+  # null means max timeout (infinity)
+  # 0 means disable timeout
+  grubEfiTimeout = if config.boot.loader.timeout == null then
+      -1
+    else
+      config.boot.loader.timeout;
+
   # The configuration file for syslinux.
 
   # Notes on syslinux configuration and UNetbootin compatiblity:
@@ -286,7 +294,7 @@ let
     if serial; then set with_serial=yes ;fi
     export with_serial
     clear
-    set timeout=10
+    set timeout=${toString grubEfiTimeout}
 
     # This message will only be viewable when "gfxterm" is not used.
     echo ""