buildroot

vsftpd: add empty directory /usr/share/empty

Vsftpd needs an empty directory where it can chroot.
If /usr/share/empty isn't present it refuses to work in the default
configuration.

Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>