Externalise boot-time prescaler value to CKDIV macro

[martincizek]

Add CKDIV macro to allow for bootloader builds with custom boot-time clock prescaler. It can be used to build bootloaders:

• for chips that would be overclocked without prescaling (e.g. 16MHz crystal and 3V Vcc)
• to reset prescaler even with 1 MHz F_CPU setting, which is currently excluded from prescaler resetting.

CKDIV name is inspired by the CKDIV8 fuse name. I can refactor it if there is a better suggestion.

Moved from MCUdude/MiniCore#163 Resolves MCUdude/MiniCore#159