Added dual extruder variant of Gen7 board

Added UBC Rapid's variant of the Gen7 board. This variant is the same as the original Gen7-AVR 1.5 board except for the following modifications:

-> Added an additional stepper motor driver which can be used for dual extrusion or other purposes. This board variant supports a total of five integrated stepper motors. Unlike the Gen7 stepper driver expansion board, the fifth stepper driver is connected to existing power rails and does not require an additional disk power connector.

-> Mounting holes around the stepper motor drivers for optional mounting of a
large heatsink or fan.

This board is designed for those who know ahead of time that they would like five stepper drivers.