tcpdump: use libpcap shared library

Currently the tcpdump configure script finds libpcap.a in
output/build/libpcap-[version]. This check is hard coded in aclocal.m4. Add a
patch to optionally ignore local libpcap so that the system shared library
version is found instead. This reduces the tcpdump binary size in a few 100KB.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>