I didn't know about this method, could have saved me much time, but the tricky part for me was to move /usr and /var loosely discussed by ruskie (I thank him a lot) and an example was given somewhere for n800 (not very usefull for n900). Now that I have moved these directories, I could install sdk (and every software i want) without worrying about rootfs or any partition. Finally, notice that my solution only requires a N900 and fingers.