WIP: Add rpi5 support to hello_snmp_agent

[pojiro]

snmpwalk -v 2c -c public nerves.local .1.3.6.1.3.17
iso.3.6.1.3.17.1.0 = INTEGER: 1
iso.3.6.1.3.17.2.0 = INTEGER: 123
iso.3.6.1.3.17.2.0 = No more variables left in this MIB View (It is past the end of the MIB tree)

TODO

• update document, snmp option or ...
• avoid target specific pin
  • ref. https://github.com/pojiro/nerves_examples/blob/f7d4c9ac21a02d3d077a3cd327a8e5039839fa09/hello_snmp_agent/lib/hello_snmp_agent/snmp/mib/my_mib.ex#L11-L27