Allow configuring a fast-clock for Teensy3.
Allow configuring a fast-clock for Teensy3.

But don't configure it by default, doesn't seem necessary.

ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter