libndp: new package

[Thomas: add patch to add stdarg.h include needed on uClibc, add
missing upstream URL in Config.in, fix license to be LGPLv2.1+ instead
of LGPLv2.1]

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>