net: apps: echo_server: Add Ethernet support

This patch adds a NET_INTF makefile configuration option to the
echo_server sample application to allow Ethernet support to be enabled
by specifying NET_INTF=ethernet.  It currently includes specific
support for the Ethernet device in the Intel Galileo.

Change-Id: I6b605db32d2d8a4259fc003724b2daa11c9a26d9
Signed-off-by: Michael LeMay <michael.lemay@intel.com>