Additional support for Durgod K320

- Cleaning up k320 keymap
- Renaming durgod_k320 => k320
- Removing copy of ST_NUCLEO64_F070RB from K320. Replacing with local board.h
- Adding Mac keyboard layout for K320 as alternative via Fn+F12
- Implementing Windows Key lock on K320
- Enhanced layer change support on K320
- Configuring MacOS layout for K320
- Adding option to change default K320 layout
- Adding Mac Media Lock functionality.  Updating readme
- Cleaning up duplicated core functionality