null: new poll mode driver

null: new poll mode driver

Null PMD is a driver of the virtual device particularly designed to measure
performance of DPDK PMDs. When an application call rx, Null PMD just allocates
mbufs and returns those. Also tx, the PMD just frees mbufs.

The PMD has following options.
- size: specify packe size allocated by RX. Default packet size is 64.
- copy: specify 1 or 0 to enable or disable copy while RX and TX.
	Default value is 0(disabled).
	This option is used for emulating more realistic data transfer.
	Copy size is equal to packet size.

To use the PMD, enable CONFIG_RTE_BUILD_SHARED_LIB in config file. Then
compile the PMD as shared library. The library can be linked using '-d'
option when an application invokes.

Here is an example.
$ sudo ./testpmd -c f -n 4 -d librte_pmd_null.so \
	--vdev 'eth_null0' --vdev 'eth_null1' -- -i --no-flush-rx

If testpmd is compiled with CONFIG_RTE_BUILD_SHARED_LIB, it may need to
specify more libraries using '-d' option.

Signed-off-by: Tetsuya Mukawa <mukawa@igel.co.jp>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>