fixes the GPIO_LOW interrupt on the atmega platform.
It results from trying to shift GPIO_LOW. Since it is 0, it is not shiftable and will not be set correctly.
There were more issues with the other flanks too, as they are 0b01 or 0b00. If 0b11 was set as a flank before it would not be able to switch to any other mode anymore. Now the bits get cleared before the new flank will be written.
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.
