Maybe skip system upgrade in raspi-blinka.py

[timonsku]

I was wondering why the script does a full system upgrade? I'm not sure if thats so clear for the users that this happens, this is a process that can potentially take hours and will touch huge parts of the system. This can have really detrimental affects on a setup, I think this should at least be question if the user wants to do this or not instead of just doing it and forcing yes on every upgrade without the user seeing what happens.

If the intention was to upgrade to a newer Raspbian maybe instead have a check for the kernel version and only suggest a upgrade or re-install if its a very old version that can't be supported any more?