Validate checksum on ROM startup

[blkerby]

It's not uncommon for players to experience problems with SD cards going bad and the ROM getting silently corrupted, resulting in crashes and other issues. We should consider adding a check on startup to verify the ROM checksum, which should be able to catch most of these issues.

[jeboo]

This would be helpful, I'm suspecting it will have to be a 'background process' to avoid adding a noticeable delay to boot-up.