extlinux-conf-builder: Set menu title to force prompt display

Without a menu title, U-Boot's distro scripts just autoboot the first
entry by default.

When I initially wrote this, my board wasn't apparently running stock
U-Boot but had some local hacks saved in the U-Boot's environment
which made it always display the prompt.

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.sh

@@ -109,6 +109,7 @@ cat > $tmpFile <<EOF
 # Change this to e.g. nixos-42 to temporarily boot to an older configuration.
 DEFAULT nixos-default
 
+MENU TITLE ------------------------------------------------------------
 TIMEOUT $timeout
 EOF