Current there is no way to split code between ATmega and ATxmega in
drivers. This differentiate AVR8 cores into MEGAs and XMEGAs.
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
Some periph_rtt implementations do not provide `rtt_set_counter()`. This
adds `periph_rtt_set_counter` as feature to allow testing for its
availability. The feature is provided at CPU level if periph_rtt is
provided by the board for all CPUs implementing `rtt_set_counter()`.
Split atmega_common code into avr8_common folder. This moves common
avr8 code to be used for all avr8 variants: tiny, mega and xmega.
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
