Import the udhcpsnoop package from the TIP feed.

The original Makefile in TIP repo spcifies GPL-2.0 as the PKG_LICENSE.
However this is a mistake, as the SPDX headers of the repos source files
all specify BSD-3-Clause as their license.

See https://github.com/Telecominfraproject/wlan-ap/blob/uCentral-trunk/feeds/ucentral/udhcpsnoop/Makefile

Signed-off-by: David Bauer <mail@david-bauer.net>

Add license information.

Small cleanups.

Signed-off-by: Rosen Penev <rosenp@gmail.com>