Added two new config options to clixon-config.yang: `CLICON_HTTP_DATA_ROOT`
Added new files: apps/snmp/snmp_mib_yang.[ch] for generic MIB/YANG handling
Test: killall quiet
Test: added specific MIB for generic code: <CLICON_SNMP_MIB>NET-SNMP-EXAMPLES-MIB</CLICON_SNMP_MIB>
