Pull up enabled by default on RPM pin

[romoloman]

OXS leave puls enabled on pins used as digital inputs like RPM pin.
Some optical sensors that uses an LM353 comparator have not enough low impedance to drive the input low, thus the rpm edge detection never happens.
I think it would be better to leave pulls disabled by adding in setuprpm function something like:
gpio_disable_pulls(config.pinRpm); //disable pullup/pulldown on rpm pin
just after having checked that the pin is defined and leaving to an external pull-up or pull-down the duty to pull the pin if necessary.