map-agent: merge different ways of doing backhaul detection
Today, we use different backhaul detection scripts:
- dynbhd for devices without dedicated WAN port and agent only
- ETH WAN check for devices with dedicated WAN port
We now also have devices without a dedicated WAN port and can have controller running. DHCP discovery method is introduced for such devices and all these 3 methods are merged into the same script for easier maintenance.