bootloader: grub: Refactor eye-candy a bit.

* gnu/bootloader/grub.scm (eye-candy)[setup-gfxterm-body]: Define the GFXMODE
binding using AND-LET* instead of chained AND=>.  Add a comment about
supporting graphical mode on other systems than x86.  Generate configuration
string using FORMAT rather than STRING-APPEND.

gnu/bootloader/grub.scm

@@ -36,6 +36,7 @@
   #:use-module (ice-9 match)
   #:use-module (ice-9 regex)
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-2)
   #:export (grub-image
             grub-image?
             grub-image-aspect-ratio
@@ -149,24 +150,26 @@ STORE-MOUNT-POINT is its mount point; these are used to determine where the
 background image and fonts must be searched for.  SYSTEM must be the target
 system string---e.g., \"x86_64-linux\"."
   (define setup-gfxterm-body
-    ;; Intel and EFI systems need to be switched into graphics mode, whereas
-    ;; most other modern architectures have no other mode and therefore don't
-    ;; need to be switched.
-    (if (string-match "^(x86_64|i[3-6]86)-" system)
-        (string-append
-         "
-"
-         (let ((gfxmode (and=>
-                         (and=> config bootloader-configuration-theme)
-                         grub-gfxmode)))
-           (if gfxmode
-               (string-append "set gfxmode=" (string-join gfxmode ";"))
-               "# Leave 'gfxmode' to 'auto'."))
-         "
+    (let ((gfxmode
+           (or (and-let* ((theme (bootloader-configuration-theme config))
+                          (gfxmode (grub-gfxmode theme)))
+                 (string-join gfxmode ";"))
+               "auto")))
+
+      ;; Intel and EFI systems need to be switched into graphics mode, whereas
+      ;; most other modern architectures have no other mode and therefore
+      ;; don't need to be switched.
+
+      ;; XXX: Do we really need to restrict to x86 systems?  We could imitate
+      ;; what the GRUB default configuration does and decide based on whether
+      ;; a user provided 'gfxterm' in the terminal-outputs field of their
+      ;; bootloader-configuration record.
+      (if (string-match "^(x86_64|i[3-6]86)-" system)
+          (format #f "
+  set gfxmode=~a
   insmod all_video
-  insmod gfxterm
-")
-        ""))
+  insmod gfxterm~%" gfxmode)
+          "")))
 
   (define (setup-gfxterm config font-file)
     (if (memq 'gfxterm (bootloader-configuration-terminal-outputs config))