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>

targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb.sa.band78.273prb.fhi72.4x4-liteon.conf

@@ -275,6 +275,7 @@ log_config :
 
 fhi_72 = {
   dpdk_devices = ("0000:31:06.0", "0000:31:06.1");
+  system_core = 0;
   io_core = 4;
   worker_cores = (2);
   du_addr = ("00:11:22:33:44:99", "00:11:22:33:44:99");