Persist eeprom to file

[pludov]

Thank for the great work !

This pull request allows to store the EEPROM data to a file. An environment variable is used (SIMAVR_EEPROM_PATH)

The file is opened using mmap so the process is transparent in the rest of the code

[buserror]

Thanks for the pull request, however I tried hard not to add that sort of features to simavr -- that's why there's the 'simduino' which /does/ the file emapping for both flash and eeprom if I remember well. Basically, you can map files before you launch core, and close them afterward...

So if you need that functionality, please use simduino as a base. I've had a branch for a more complete tool to replace simduino in the work for a long time (haven't had time to dedicate time to it in a logn time too!) -- check the branch wip-shell for an idea...