SDCard: Initial support for refreshing file list / exchange of SDCard 

This change set implements a remount command in simpleshell and M21.
- This permits to exchange SDCards in smoothieware while running. 
- This allows to write from USB and see the changes locally.

It does not handle concurrent writing (so no change here). Remount will not flush the internal cache to card, it discards it.
To be on the safe side, stick to the rule write either from USB only or local only.