I have read on this forum and some months ago (not too many) that someone (at least two users) has created a u-boot initrd to boot Nitdroid successfully. It was not completed detached from the usage of rootfs because Nitdroid still wants to load some firmwares from rootfs after booting process, but it was however a big step forward. Unfortunately they haven't published their works but if I remember correctly their posts were on BackupMenu thread