Revert "Fix 12v voltage rail sensor sysfs name"

This reverts commit fc068eadfa1cd7d7ea7e49fe7c459a756c9a7e41.
Add scaling factors for currents and 12V voltage

This adds default scaling factors for the current sensors and the 12V
voltage sensor.  These default scaling factors are for ROACH2 rev2.
They can be overridden in /etc/tcpborphserver3.init.
Add sample tcpborphserver3.init file
Add tags to .gitignore
Deprogram FPGA if progdev is not given a BOF name

Calling progdev without a BOF filename was a no-op.  The progdev command
now treats a missing filename as a request to deprogram the FPGA (e.g.
to save power).  Prior to this, there was no way (that I knew of) to
explicitly deprogram the FPGA via KATCP.