bootloader: grub: Skip install-grub-efi when producing a disk image.

Fixes <http://issues.guix.gnu.org/44353>.

Every bootloader should try their best to install themselves using only the
MOUNT-POINT and otherwise do nothing.  This requirement comes from the
necessity to call INSTALL-GRUB when installing the (non-EFI) GRUB bootloader,
which needs to populate the root file system with extra modules that cannot be
fit in the core.img file, limited in size to 491520 bytes (by the i386-pc
format required for legacy BIOS compatibility).

As introducing bootloader knowledge at the level of the image code is
undesirable, every bootloader should be adapted to support this fall-back for
their installation procedure (TODO).

* gnu/bootloader/grub.scm (install-grub-efi)[efi-dir]: Skip when the EFI-DIR
argument is set to #f.

gnu/bootloader/grub.scm

@@ -573,6 +573,9 @@ fi~%"))))
 
 (define install-grub-efi
   #~(lambda (bootloader efi-dir mount-point)
+      ;; There is nothing useful to do when called in the context of a disk
+      ;; image generation.
+      (when efi-dir
         ;; Install GRUB onto the EFI partition mounted at EFI-DIR, for the
         ;; system whose root is mounted at MOUNT-POINT.
         (let ((grub-install (string-append bootloader "/sbin/grub-install"))
@@ -587,7 +590,7 @@ fi~%"))))
           (setenv "GRUB_ENABLE_CRYPTODISK" "y")
           (invoke/quiet grub-install "--boot-directory" install-dir
                         "--bootloader-id=Guix"
-                      "--efi-directory" target-esp))))
+                        "--efi-directory" target-esp)))))
 
 (define (install-grub-efi-netboot subdir)
   "Define a grub-efi-netboot bootloader installer for installation in SUBDIR,