Add more jumpers for Teensy++ 2.0 compat

This avoids requiring folks to pull pins out of headers for Teensy++ 2.0
compatibility, instead just requiring them to cut the traces, which I
think is better and more importantly is reversible, so that if someone
builds with a Teensy++ 2.0 and then wants to replace it with a 3.6, they
can just re-close the new jumpers, desolder the old ones, and be good to
go.