* Add support for U2X switch to double transmission speed
* Provides lower error rates when needed
* Add support for static baud rate calculation, reduces computation by 10x
* adds support for statically enabling double speed baud
use generic avr/libc definition for conditionnal compilation.
eg: use #ifdef(PORTG) or #ifdef(TIMER_0_ISRC) instead of #ifdef(Atmega328p)
This is more generic and simplify future boards support.
Makes AVR register definitions dependent on what avr-libc defines
for a given MCU, rather then duplicating that effort here.
Definitions done in this way are based on functionality provided,
rather than a specific MCU device.
Hauke Petersen