O-RAN 7.2: make DPDK system core configurable

system_core is the core reserved for DPDK control threads. It was
hardcoded to 0, but it seems that one some systems, CPU 0 cannot be
reasonably isolated from kernel threads. Make it therefore configurable
(default: 0)

Closes: #732 (suggestion 1)
Suggested-by: Andrew Sergeev <andrew.sergeev@adtran.com>