net-snmp: add uci section to include additional conf files

[rthakur33]

net-snmp allows including additional conf files by specifying then with the includeFile token in the snmpd.conf, this commits adds a uci section that translates to includeFile in snmpd.conf files and hence allows including additional conf files.

the syntax introduced is as follows:

config include 'custom-file' option path '/tmp/mysnmpd.conf'