grub2|loopback: avoid empty submenu

don't include the advanced menu at all if it's only entry - memtest - is
not included (you just get a menu entry that does nothing, which may
confused users into thinking that something is broken, as opposed to
showing an empty submenu as i expected).

Gbp-Dch: Short

share/bootloaders/grub-pc/grub.cfg

@@ -57,11 +57,11 @@ submenu 'Advanced install options...' {
 }
 fi
 
+if @ENABLE_MEMTEST@; then
 submenu 'Advanced options...' --hotkey=a {
 
 	# Memtest (if any)
-	if @ENABLE_MEMTEST@; then
 	source	/boot/grub/memtest.cfg
-	fi
 
 }
+fi