mkfs-util: Skip non files/directories when calling mcopy

mkfs-util: Skip non files/directories when calling mcopy

Only files and directories are supported by vfat. When we pass a
symlink to mcopy, it will try to dereference them and copy what the
symlink points at into the vfat partition instead. Let's avoid this
by skipping all unsupported file types when establishing the list of
top level targets that mcopy should copy.

We also use RECURSE_DIR_SORT everywhere when iterating directories
to make things more reproducible.