dpdk: add support for v2.1.0

dpdk: add support for v2.1.0

Update relevant artifacts to add support for DPDK v2.1.0
 - INSTALL.DPDK.md
 - acinclude.m4: Change DPDK library name
 - netdev-dpdk: Limit minimum mbuf size to to adapt to DPDK bug fix that
   changes the treatment of the requested mbuf size
 - build.sh: Change DPDK version number

Note that this breaks compatibility with DPDK v2.0.0 although only
for the library name change.

Note that throughput for vhost ports with mergeable buffers is reduced
about 10% due to a necessary bug fix in DPDK vhost code.

Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Signed-off-by: Michal Weglicki <michalx.weglicki@intel.com>
Signed-off-by: Timo Puha <timox.puha@intel.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>