Well, try to download recent version of the flasher, there is README and example configurations, that may answer your questions.

Now you can have optional bootmenu.conf along with bootmenu.sh. Real versions which are used while booting reside in /mnt/initfs. Either you can create it in flasher directory and reflash or edit directly in /mnt/initfs (which may produce disk full errors on N770 if you do it multiple times, check the README)