bug#39095: GRUB EFI should have a 'chainloader' option

[symphonia]

To dual boot with NixOS and other operating systems it would be nice to have 
'chainloader' so that this configuration would work:
(bootloader
    (bootloader-configuration
      (bootloader grub-efi-bootloader)
      (target "/boot/efi")
      (menu-entries
       (menu-entry
        (label "systemd-boot")
        (chainloader "/EFI/Boot/bootx64.efi")))