Correcting and clarifying the documention for how to build OVS images
with DPDK support:
* The additional heading was confusing and the following sentence even
more so.
* Fixed the image name in the build command.
* Updated the OVS and DPDK version numbers to current stable versions.
Change-Id: Ib12be3da4f1fc14f1fed990a8abcdf87477a4e1b