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.
- Created new `bluepill-128kib` as 128KiB version of the `bluepill`
- Created new `blackpill-128kib` as 128KiB version of the `blackpill`
- Updated `openocd.cfg` to allow flashing 128KiB of ROM