sys.sysinfo() call in system plugin gives a warning

[rogerpueyo]

When running "libremap-agent" on recent OpenWrt-based devices (e.g. Chaos Calmer 15.05.1), the following warning is logged:

Thu Sep  1 14:59:25 2016 user.warn libremap-agent[23499]: unable to execute plugin "system"; ?:0: attempt to call field 'sysinfo' (a nil value)
Thu Sep  1 14:59:26 2016 user.info libremap-agent[23499]: success: submitted to http://libremap.net/api/