Modify tcpdump options to add interface mask

Added a mask parameter to tcpdump option. This extra parameter allows
users to select which interface traffic will be captured.

Proposed mask values:

- 1 to select eth0 (regular interface)
- 2 to select eth1 (inter-emulators interface)
- 3 to select both interfaces

Change-Id: Ifd7bbf8c9793ac8b0e4cf40edca90a9f9e8ebe1e