examples/ntb: add example for NTB

examples/ntb: add example for NTB

Enable an example for rawdev ntb. Support interactive mode to send
file on one host and receive file from another host. The command line
would be 'send [filepath]' and 'receive [filepath]'.

But since the FIFO is not enabled right now, use rte_memcpy as the enqueue
and dequeue functions and only support transmitting file no more than 4M.

Signed-off-by: Xiaoyun Li <xiaoyun.li@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>
Reviewed-by: Xiaolong Ye <xiaolong.ye@intel.com>