[config] Generate sysinfo in single asic (sonic-net#2856)

What I did
It is a bug introduced from sonic-net#2836. Need to generate sysinfo for single asic.

How I did it
Reuse the mac and platform in existing device runnning config and generate that if missing.

How to verify it
Unit test

config/main.py

@@ -1906,10 +1906,10 @@ def override_config_table(db, input_config_db, dry_run):
  ns_config_input = config_input["localhost"]
  else:
  ns_config_input = config_input[ns]
- # Generate sysinfo if missing in ns_config_input
- generate_sysinfo(current_config, ns_config_input, ns)
  else:
  ns_config_input = config_input
+ # Generate sysinfo if missing in ns_config_input
+ generate_sysinfo(current_config, ns_config_input, ns)
  updated_config = update_config(current_config, ns_config_input)
 
  yang_enabled = device_info.is_yang_config_validation_enabled(config_db)