Write an upgrading utility that runs separate to netsender.

Currently our iot/pi/netsender client is the program responsible for managing a remote software upgrade. This should change so that netsender triggers another program to manage the upgrade of the software since netsender itself may need upgrading. A program should not be responsible for upgrading itself.

This way a fallback version as described in iot issue 71 can be implemented in a safer way.