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.
Currently only tested boards provide the feature riotboot.
Potentially all boards embeding a cortex-m0+/3/4/7 are
able to have riotboot as a feature, but other dependencies
need to be met, e.g. usage of cortexm.ld linker script,
double initialisation of cpu_init(), etc. See doc in
bootloaders/riotboot.
