Boy with Staff and Goose
A boy with a staff carrying a large bird that is probably a goose under his left arm. The emblem might be based on the Grimm fairy tale, The Golden Goose.
In that fairytale there is a young man named "Simpleton" who wants to marry the princess of an empire. Her father, the king, gives him three trials to accomplish. The last trial is to find a ship which can also sail both on water as on land.
This fits the history of U-20 perfectly as she was indeed "sailed" over land to reach the Black Sea.
1 U-boat displayed this emblem