Eating in public is basically totally ok in Berlin. You can either walk down the street with a Döner Kebab in your hands or sit down in a park, on a bench and have a Pizza. Both is fine.
Basically you can even have a full blown picnic in public – but you should probably consider not having your picnic right at the next street corner. You can instead go to a nearby park, spread a blanket, open a bottle of wine (drinking in public is allowed too!) and have your meal.
In many parks across the city you can even your own barbecue. Just please (Please!) do clean up after you’re done! There are trash bins everywhere and we please don’t want the next picnic to be spoiled by your litter. Or would you enjoy looking for a green spot in a park that’s covered with picnic-leftovers? Probably not.
Exceptions: eating in public transport is prohibited.
see also: Drinking in Public