Generate per-MCU port definitions

This was mainly done to make it easier to use arduino-- in other projects.

Previously, avr-ports.h was generated depending on the MCU, now, all supported
MCU port definitions are available upon checkout, which simplifies the Makefile
in projects depending upon arduino--

Also reworked was the logic for the Timer0 prescaler. The Timer0 prescaler
can now optionally be set in Makefile.local and the Timer::micros function
should work correctly across all possible values.