[faiproject/fai] 412c9c: fix syntax error, grub test is ==

[Mrfai]

  Branch: refs/heads/master
  Home:   https://github.com/faiproject/fai
  Commit: 412c9c06be6b4432904b66fa3f92445ec7fbfad7
      https://github.com/faiproject/fai/commit/412c9c06be6b4432904b66fa3f92445ec7fbfad7
  Author: Thomas Lange <lange at debian.org>
  Date:   2023-12-02 (Sat, 02 Dec 2023)

  Changed paths:
    M conf/grub.cfg
    M conf/grub.cfg.autodiscover

  Log Message:
  -----------
  fix syntax error, grub test is ==