As after a few trials I succeeded to root my device, let me share the procedure I followed to achieve that.
Note - Requires ADB installed on your PC.
If root fs is not mounted as read/write, these preliminary (critical) steps are needed:
0.1) adb pull /init.rc init.rc.tmp
Then, edit locally init.rc.tmp, looking for "mount rootfs rootfs / ro remount" line and change "ro" to "rw".
0.2) adb shell "mount -o rw,remount -t ext3 /dev/root /"
0.3) adb push init.rc.tmp /init.rc
0.4) adb shell chown sytem:system /init.rc
0.5) adb shell 644 /init.rc
Reboot. If anything goes wrong and rebooting fails, you have to re-flash the ROM.
Once /system is writable, the "su" hacking goes like this:
1) Install SuperSU app (by 'Chainfire'), from Google Play
2) Trying to run the su binary update will fail
3) Download UPDATE-SuperSU-v2.00.zip from
http://download.chainfire.eu/450/SuperSU/ and extract to your PC, then go to that folder
4) adb shell mkdir /system/bin/.ext
5) adb shell push arm/su /system/bin/.ext/.su
6) adb shell chmod 6755 /system/bin/.ext/.su
7) adb shell push arm/su /system/xbin/su
8) adb shell chmod 6755 /system/xbin/su
9) verify with:
adb shell
root@android:/ # su 1000
system@android:/ $ su ---> here SuperSU app should prompt for a confirmation request
root@android:/ #
10) Say thanks to Chainfire by purchasing SuperSU pro, if you like... and let us know if you also made it...