stm32f1 periph_rtc implementation gets a 1s resolution by dividing
CLOCK_LSx by 32768. This only make sense if CLOCK_LSE is set,
otherwise CLOCK_LSI=~40000, which will lead to an imprecise rtc.
cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
The file always exist so no need to do '-include'.
Replaced using:
sed -i 's|-\(include $(RIOTCPU)/.*/Makefile.features\)|\1|' \
$(git grep -l '$(RIOTCPU)/.*/Makefile.features' boards)