This is strictly from P3Droid

Doing the following will allow your device to maintain root through any upgrade (unless motorola changes things)

This works on all moto devices. It has been said that you should start from a stock (if on Bionic 5.5.886) then root via Pete’s RootTools

To get to stock check here       To Root your Bionic

I use root explorer, but you can also use adb pull and push.

RootExplorer Method

go to /system/bin/
open in text editor – mount_ext3.sh
add the following lines to the very bottom

chmod 4755 /system/bin/su
chmod 4755 /system/xbin/su
chmod 4755 /system/app/Superuser.apk

ADB method

adb pull /system/bin/mount_ext3.sh
open the file in your favorite text editor
add the following lines to the very bottom[INDENT]
chmod 4755 /system/bin/su
chmod 4755 /system/xbin/su
chmod 4755 /system/app/Superuser.apk[/INDENT]
Save file

adb push mount_ext3.sh /data/local
adb shell
mount -o rw,remount /dev/null /system
cat /data/local/mount_ext3.sh > /system/bin/mount_ext3.sh
chmod 777 /system/bin/mount_ext3.sh

Done!!!

This method was proposed by Djrbliss, and only slightly altered by me…please thank @DjrBliss on Twitter.

How to maintain root through any upgrade (Motorola Devices Only)

Thanks DjrBliss!
Thanks P3Droid!