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>

doc/ORAN_FHI7.2_Tutorial.md

@@ -601,6 +601,8 @@ Edit the sample OAI gNB configuration file and check following parameters:
   that is employed by the xRAN library (`xran_fh_init` and `xran_fh_config`
   structs in the code):
   * `dpdk_devices`: PCI addresses of NIC VFs binded to the DPDK
+  * `system_core`: absolute CPU core ID for DPDK control threads
+    (`rte_mp_handle`, `eal-intr-thread`, `iavf-event-thread`)
   * `io_core`: absolute CPU core ID for XRAN library
   * `worker_cores`: array of absolute CPU core IDs for XRAN library
   * `du_addr`: DU C- and U-plane MAC-addresses (format `UU:VV:WW:XX:YY:ZZ`,