How can I make persistent changes in root directory (/)? I have to edit my fstab in android 4.4 but the root is mounted in read only.
I've try to remount the root with
mount -o rw,remount /And I can edit the root files but when I reboot the device the changes disappear. 
How can I make persistent changes in / without rebuild the kernel?
For that you'd need to edit the corresponding image (see initrd), which is unpacked on each boot.

Alternatively, you could create a script replacing the fstab file, and have this run on each boot e.g. using tasker or a similar app that can execute "stuff" automatically after boot.