Delete dist dir if running distribute.sh

This pr makes buildozer delete the existing p4a distribution if it is going to run distribute.sh

This is important because p4a raises an error message/question if the distribution already exists, but unless log_level=2 this message is *not* displayed by buildozer, so it looks like buildozer is hanging.

I don't know if this is the best way to solve the problem, but something needs to be done to fix the bug, and I couldn't think of anything better.