samples: net: Fix the 802.15.4 monitor pcap saving

By default the net-tools package is expected to be located in
${ZEPHYR_BASE}/../net-tools directory. User can also specify
the directory using NET_TOOLS variable when running the make.

The net-tools package is located in this repository
https://gerrit.zephyrproject.org/r/net-tools



Change-Id: Ibccd7cabd567a630020fb9efbe1ec9e27b653b46
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>