Add configurable DIRECTORY_NAME for rclone (Now compatible with S3 providers)

[BestPig]

For S3 provider, the destination bucket is not configurable in rclone.conf.
With old script, MiSTer is used as a bucket name.
With need a way to configure it, because Amazon S3 bucket are unique across all existing AWS bucket (not per user), and bucket name must be in lower case (event with self hosted solution like minio).

I replaced

MiSTer:MiSTer/$RCLONE_SD_DIR"

with

MiSTer:$DIRECTORY_NAME/$RCLONE_SD_DIR"

And made DIRECTORY_NAME configurable in a rclone.ini, if not exists, the default value MiSTer is used.
So no breaking change.
The update also include a documentation update to explain how to define this value.