List @ filesystems in root directory listing

[peterbarker]

• dirlist now works
• Callisto model (which relies on a file in ROMFS) still works
• can still fetch a file using ftp get

[tridge]

I wonder if we should add the @ filesystems that support directory listing to root of / ?

[peterbarker]

I tested this in SITL, both MAVProxy and MissionPlanner, and on CUAVv5, both MAVProxy and MissionPlanner.

[IamPete1]

Just tested on mission planner, this means we can remove the @SYS special case it uses.

To work fully we would also need to patch @MISSION and @PARAM to list files, currently they just appear empty.

[tridge]

we should have a bool supports_dirlist() method on the backend, false for MISSION and PARAM until we add those