This was previously configured to PC5 which is shared with the i2c
SCL line. PD7 is selected since it's the only pin which does not
have an additional function.
Refactors boards/arduino-common to boards/arduino-atmega-common.
Extends boards/arduino-atmega-common to be generic to all atmega based
arduino boards, not just atmega328p.
Removes flags that should be located in cpu framework