Adaption of LCD info screen to # of extruders

Here comes a quick compile-time operation to match the extruder symbols on the info screen to the set number of extruders in configuration.h.
When only one extruder is selected, the "1" on the symbol will not be displayed.

Aditionally I removed the "---" for unknown temperature values when using 1 or 2 extruders.

![fgscrrd1](https://cloud.githubusercontent.com/assets/9697253/5013622/8d6be3e8-6a8f-11e4-8f1e-c93b516c9438.jpg)