Change filename of cached core to bypass Windows length limitations

[facchinm]

As reported on arduino/Arduino#7902 , if a core has many build flags the cached core filename ends up being too long (more than 255 characters).
Two possible solutions:

• adding an index file (containing all the needed information) (0% chance of clashing filenames)
• hashing the actual filename (with sha256 for example) and using the hash as filename (very low chance of name clashing)

[matthijskooijman]

A combination might also make sense: Use a hash to quickly locate a cached core, and put the full information in a file inside the cached core to verify. In the unlikely event of a hash conflict, this allows wiping the previous cache, rather than getting a broken build. Having such a file is a good idea in any case, since it allows a user to inspect a cached directory and see what it is for as well.

[JiriBilek]

Hi, just a comment to the hash collision threat. Having only a few hashed strings we can safely ignore it. See e.g. http://preshing.com/20110504/hash-collision-probabilities/probabilities/ (the table at the very bottom).