How to root a Samsung I9070 Galaxy S Advance/S II Lite? What are the steps to root this phone?

As I understand, Samsung has been using an internal counter that will void your guarantee if you install a custom kernel, I'd like to avoid that.
Solution for at least I9070 / I9070P on Linux:

Flashing only system.img with Odin or Heimdall (use 1.3.1 version) does
not increase the counter.

So download an official stock ROM for your phone, extract it, 
convert the system.img (sparse ext4 image) to an normal ext4 image as described here. Loop-mount it, add a su binary to the xbin subfolder with the correct file rights and ownership (06755 root root)  and if necessary add other files which are on your phone in the /system folder but not in the stock rom. Repack everything back to a sparse ext4 image and flash it.

At the end install the Superuser or SuperSu app which belongs to the su binary you added to the system image.