doc: Give an example with a FAT UUID.

* gnu/system/examples/lightweight-desktop.tmpl <file-systems>: Add a
UUID for the /boot/efi partition.
* doc/guix.texi (Using the Configuration System): Mention it.

doc/guix.texi

@@ -8406,6 +8406,7 @@ management, power management, and more, would look like this:
 @include os-config-desktop.texi
 @end lisp
+@cindex UEFI
 A graphical UEFI system with a choice of lightweight window managers
 instead of full-blown desktop environments would look like this:
@@ -8413,6 +8414,10 @@ instead of full-blown desktop environments would look like this:
 @include os-config-lightweight-desktop.texi
 @end lisp
+This example refers to the @file{/boot/efi} partition by its UUID,
+@code{1234-ABCD}.  Replace this UUID with the right UUID on your system,
+as returned by the @command{blkid} command.
+
 @xref{Desktop Services}, for the exact list of services provided by
 @var{%desktop-services}.  @xref{X.509 Certificates}, for background
 information about the @code{nss-certs} package that is used here.

gnu/system/examples/lightweight-desktop.tmpl

@@ -17,16 +17,16 @@
                 (bootloader grub-efi-bootloader)
                 (target "/boot/efi")))
 
-  ;; Assume the target root file system is labelled "my-root".
+  ;; Assume the target root file system is labelled "my-root",
+  ;; and the EFI System Partition has UUID 1234-ABCD.
   (file-systems (cons* (file-system
                          (device "my-root")
                          (title 'label)
                          (mount-point "/")
                          (type "ext4"))
                        (file-system
-                         ;; Specify partition here since FAT
-                         ;; labels are currently unsupported.
-                         (device "/dev/sda1")
+                         (device (uuid "1234-ABCD" 'fat))
+                         (title 'uuid)
                          (mount-point "/boot/efi")
                          (type "vfat"))
                        %base-file-systems))