This function was a duplication of the existing gpio_init_mux,
where the exiting function is the more general one, as the mux
inititalization is not only valid for the sercom module...
The former i2c_init_master was too complicated. It was trying to set IN/OUT
mode of the pins, but all that is needed is to set the proper MUX.
Also the configuration for the boards was incomplete (no MUX, no pad
setting).
It was tested on a SODAQ Autonomo, but not on a samr21-xpro