void ctl_dns_set_secondary_server_addr(CTL_NET_IPv4_ADDR_t addr);
ctl_dns_set_secondary_server_addr sets the secondary DNS server IP address to addr.
Other parts of the network library may overwrite the address set by this function, for instance when DHCP negotiation is complete.
ctl_dns_set_primary_server_addr and ctl_dns_set_secondary_server_addr are decoupled from the rest of the DNS resolver so that you can use DHCP with assigned DNS server addresses set without automatically pulling in the resolver code.