So I was trying to achieve this with a 3rd gen Moto G. It has a charge_only_mode file as per Rohit's answer, but simply moving/renaming it did not make the phone reboot on charge. Instead it just sat there with a Motorola logo. I got the same result when replacing charge_only_mode with either of the scripts referenced here.
I did get it to work, however. I copied /system/bin/reboot into /system/bin/charge_only_mode, and that did the trick.
