Alternative to #7882. If F_CPU is greater than 1000 it can be evenly divided by 8. Over 10000, 16; over 100000, 32; over 1 million, 64; etc.
Adding files for STM32F1 HAL based on libmaple/stm32duino core. Current persistent_store uses cardreader changes to be sent in separate commit, but could be changed to use i2c eeprom.