Set bootmenu item file as default first entry
Create symlink to /etc/default/bootmenu.item from your item file. For example:
$ ln -s /etc/bootmenu.d/20-Maemo5-kernel-power-2.6.28.10-power51.item /etc/default/bootmenu.item

Item config file has same format as Fanoush bootmenu and is stored in same folder (Multiboot package has same format too, but in folder /etc/multiboot.d/)

After creating config run tool u-boot-update-bootmenu. u-boot-update-bootmenu must be called every time, when config file is changed.