Recursively move files from subfolders to current folder

flat_folders.sh

Transforms:
./Notification/Black@2x.png
./Notification/Black.png
./Sign out/Black.png
./Sign out/Black@2x.png
To:
.Notification_Black@2x.png
.Notification_Black.png
.Sign out_Black.png
.Sign out_Black@2x.png

To test it out:
find . -type f -exec bash -c 'file=${1#./}; echo mv "$file" "${file//\//_}"' _ '{}' \;

To perform:
find . -type f -exec bash -c 'file=${1#./}; mv "$file" "${file//\//_}"' _ '{}' \;

from:
https://unix.stackexchange.com/a/178056