If the same callback function is used for multiple interfaces
(`NETDEV_INDEX_ANY`), it is necessary to also provide the index of
the interface to hand out and address.
Every ATmega board that can run an Real Time Timer can also run
an emulated Real Time Clock.
Got all supported boards by adding `FEATURES_REQUIRED += arch_8bit`
to `tests/periph_rtt` and running
sed -i 's/FEATURES_PROVIDED += periph_rtt/
FEATURES_PROVIDED += periph_rtc\n
FEATURES_PROVIDED += periph_rtt/g'
on them.
