[spp] Create new branch for SPP v18.08

[Yasufumi Ogawa <yasufum.o@gmail.com>]

Hi all,

Version of SPP is still v18.08.x although DPDK is v19.08, and going to 
update to v19.11. Vhost PMD of DPDK v18.11 or later vhost does not work 
for secondary process because of re-design of multi-process model. In 
addition, PMDs other than vhost has also some problems for running with 
secondary process. So, SPP has developed based on DPDK v18.08 to keep 
supporting mainly vhost, and optionally supported v18.11 or later.

However, DPDK has continued to add or revise features and SPP cannot 
compile with DPDK v19.08 because some basic data structures are changed.
To fix the problem, we need to make a change for which SPP cannot 
compile with DPDK v19.05 or previous.

I think now is the time to update SPP for the latest DPDK. In addition, 
"multi-process rework" is listed up in roadmap for v19.11 although this 
rework is one of "Nice to have - Future" [1].

I would like to continue to support v18.08.x by creating a new branch 
for supporting vhost and make updates to the branch until the latest 
DPDK works fine in multi-process at least.

[1] https://core.dpdk.org/roadmap/

Regards,
Yasufumi