net: apps: Provide pcap support for echo apps

Provide pcap support for echo-apps with PCAP=<filename> option.

For pcap support:
make server NET_IFACE=qemu PCAP=sample.pcap.

without pcap support:
make server NET_IFACE=qemu

Change-Id: Iad90064e0c32134f9c2fda7b28e2cbca1e4e931c
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>