iso-image: Make graphical output work properly on AArch64

The serial output (but it's named console, not serial actually) causes
issues on U-Boot's EFI, at the very least.

This is inspired by OpenSUSE's approach:

 * https://build.opensuse.org/package/view_file/Base:System/grub2/grub2-SUSE-Add-the-t-hotkey.patch

Where they add a hidden menu entry, which can be used to force the
console output.

The `echo` will be visible on the serial terminal (grub "console"),
while the graphical interface is shown. Note that input in the serial
terminal (grub "console") will continue controlling the graphical
interface. Useful if you have an SBC connectedinto an HDMI monitor, but
no keyboard connected to it.

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -187,6 +187,9 @@ let
     # Fonts can be loaded?
     # (This font is assumed to always be provided as a fallback by NixOS)
     if loadfont /EFI/boot/unicode.pf2; then
+      set with_fonts=true
+    fi
+    if [ "\$textmode" != "true" -a "\$with_fonts" == "true" ]; then
       # Use graphical term, it can be either with background image or a theme.
       # input is "console", while output is "gfxterm".
       # This enables "serial" input and output only when possible.
@@ -264,6 +267,8 @@ let
 
     cat <<EOF > $out/EFI/boot/grub.cfg
 
+    set with_fonts=false
+    set textmode=false
     # If you want to use serial for "terminal_*" commands, you need to set one up:
     #   Example manual configuration:
     #    → serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
@@ -273,8 +278,28 @@ let
     export with_serial
     clear
     set timeout=10
+
+    # This message will only be viewable when "gfxterm" is not used.
+    echo ""
+    echo "Loading graphical boot menu..."
+    echo ""
+    echo "Press 't' to use the text boot menu on this console..."
+    echo ""
+
     ${grubMenuCfg}
 
+    hiddenentry 'Text mode' --hotkey 't' {
+      loadfont /EFI/boot/unicode.pf2
+      set textmode=true
+      terminal_output gfxterm console
+    }
+    hiddenentry 'GUI mode' --hotkey 'g' {
+      $(find ${config.isoImage.grubTheme} -iname '*.pf2' -printf "loadfont /EFI/boot/grub-theme/%P\n")
+      set textmode=false
+      terminal_output gfxterm
+    }
+
+
     # If the parameter iso_path is set, append the findiso parameter to the kernel
     # line. We need this to allow the nixos iso to be booted from grub directly.
     if [ \''${iso_path} ] ; then