needs checking with AP_Filesystem and MAVFtp

Make sure folders can be listed by returning DT_DIR/DT_REG type.

This now allows full dir listing of ROMFS. It mostly works as any other file system. The mostly part is that we get a directory for each file. For example:

ftp list @ROMFS:

Listing @ROMFS
 D models
 D models
 D models
 D models
 D models
 D scripts
 D scripts
 D scripts
   locations.txt        4187

ftp list @ROMFS/models:

Listing @ROMFS/models
   Callisto.json        1460
   plane-3d.parm        874
   plane.parm   1739
   xplane_heli.json     1935
   xplane_plane.json    1905
Total size 7.73 kByte

So we get 5 copy's of the models directory because there are 5 files in it.

To only show each directory once we would have to stash the last directory name somewhere and skip if there is no change. I'm not sure if its worth it....

Some related PRs: #21959 #23294 #19865

To only show each directory once we would have to stash the last directory name somewhere and skip if there is no change

we can do that in dir_list() by looking at the entry at ofs-1

@IamPete1 I've fixed it up here:
https://github.com/tridge/ardupilot/commits/ROMFS_dir_list/