Merge Dev Image and Normal Image

[jfisbein]

In stead of having 2 different images, the project could have only one image with one flag checked on boot, like they do on Raspbian to enable ssh (See point 3 "Enable SSH on a headless Raspberry Pi (add file to SD card on another machine)" on https://www.raspberrypi.org/documentation/remote-access/ssh/).

The presence of this file on the boot partition (ex: telraam-dev), that should be added manually on the sd-card, will be used by all the scripts to adequate their functionality.
Example:

• If the file telraam-dev is present do not disable ssh-server
• If the file telraam-dev is present do not change the pi user password en every boot
• etc