External storage : add an option to sync when switching from internal to external
When users switch to a USB key/drive and their SD was bloated of roms, they have to follow https://github.com/recalbox/recalbox-os/wiki/Use-an-external-usb-storage-device-on-recalbox-(EN)#optional--sync-your-usb-key-with-your-sd => Syncing should be made available from ES