Certain pins (eg 4 and 5) seem to behave differently at the hardware level
when in open-drain mode: they glitch when set "high" and drive the pin
active high for a brief period before disabling the output driver. To work
around this make the pin an input to let it float high.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.