app/testpmd: add mempool with external data buffers

app/testpmd: add mempool with external data buffers

The new mbuf pool type is added to testpmd. To engage the
mbuf pool with externally attached data buffers the parameter
"--mp-alloc=xbuf" should be specified in testpmd command line.

The objective of this patch is just to test whether mbuf pool
with externally attached data buffers works OK. The memory for
data buffers is allocated from DPDK memory, so this is not
"true" external memory from some physical device (this is
supposed the most common use case for such kind of mbuf pool).

The user should be aware that not all drivers support the mbuf
with EXT_ATTACHED_BUF flags set in newly allocated mbuf (many
PMDs just overwrite ol_flags field and flag value is getting
lost).

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>