The "community" string is public. We have tree items only: the host
name, the date and uptime.

The code is designed to make it simple any addition. But we only have
snmpget support, not snmpwak (i.e. "get next" query).

This is run on an host build (CONFIG_HOST_PROCESS) where the time and
date are not related to the real world:

   # snmpget -v 1 -c public 192.168.16.1 .1.3.6.1.2.1.1.5.0
   SNMPv2-MIB::sysName.0 = STRING: wrc
   # snmpget -v 1 -c public 192.168.16.1 .1.3.6.1.2.1.25.1.2.0
   HOST-RESOURCES-MIB::hrSystemDate.0 = STRING: 1970-0-1,19:52:34.0
   # snmpget -v 1 -c public 192.168.16.1 .1.3.6.1.2.1.25.1.2.0
   HOST-RESOURCES-MIB::hrSystemDate.0 = STRING: 1970-0-1,19:53:2.0
   # snmpget -v 1 -c public 192.168.16.1 .1.3.6.1.2.1.25.1.1.0
   [...] hrSystemUptime.0 = Timeticks: (94193300) 10 days, 21:38:53.00
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>