sandbox/fs: Set correct filetype for unknown filetype

The "hostfs ls" command prefixes each directory entry with either DIR,
LNK or "   " if it is a directory, symlink resp. regular file, or
"???" for any other or unknown type.
The latter only works if the type is set correctly, as the entry defaults
to OS_FILET_REG and e.g. socket files show up as regular files.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Simon Glass <sjg@chromium.org>