I2C 'HACK' solder jumper may not work as intended due to level shifters

[securelyfitz]

The I2C 'HACK' Solder jumper shorts TDI and TDO to make a bidirectional SDA line. This is what's recommended by the FTDI datasheet, but that datasheet does not consider our usage of level shifters.

TDI and TDO both have 100ohm series resistors; bridging the 'HACK' does combine them but will have ~25ma constantly flowing between the output and input level shifters. This complicates the already difficult I2C pullup design, and requires stronger output driving from the target to overcome.

Current recommendation is to not use the HACK solder jumper.
Documentation should be revised to reflect that.
Future design changes to accomodate #52 might mitigate this issue.