Ability to switch WiFi network off and connect to another WiFi network

[jywarren]

Ideally we'd like to do this by writing into a config file, so it could be:

• done on the SD card
• done at build time with a random number as in Ability to insert random number or string into config files like WiFi #23
• done by entering WiFi info via the web interface

@xose were you able to do this on yours?

Also note connection to: Allow connection to additional WiFi network with internet access, while still providing hotspot #34