package/dropbear: rework license information

According to the LICENSE file curve25519-donna is licensed under
BSD-3-Clause license.

There is only BSD-2-Clause license mentioned so remove
BSD-2-Clause-like.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>