You'll need to edit MeeGo's boot.scr (wonder why the hell this isn't done in the first place: u-boot bootfiles in vfat p1, MeeGo in ext3/4/btrfs p2, and swap on p3, to make it play nice with Maemo) If you want to make the vfat partition something other than what's on the downloadable image, just download the u-boot for n900 sources, change which partition it points to, build, append and flash.