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.
- No longer include `tools/serial.mk` and `tools/avrdude.mk`, as this is
no done by `boards/common/atmega`
- No longer provide `RESET ?=`, as this is now done in `tools/avrdude.mk` for
all ATmega boards unless they are using a bootloader
