don't use underscore ('_') in hostname

[andbararm]

While testing the example on my Fritz!Box, I noticed, I can't connect to the application via the "traffic_light" hostname. Checking this showed the Fritz!Box had converted the underscore to a hyphen-minus sign, resulting in "traffic-light".
As the Fritz!Box acts as DNS server, it probably converts this, as DNS hostnames are not allowed to contain underscores in the name:

https://en.wikipedia.org/wiki/Hostname

NetBIOS might does allow underscores. But it should probably be changed to a hyphen-minus character to avoid such trouble.

[MiloradCvjetkovic]

Can you @andbararm , raise the same issue on the https://github.com/ARM-software/MDK-Middleware repo?