Make ipfixprobe more user friendly and bulletproof (enable it by default, better IP handling)

[BKPepe]

Proposed ideas:

1. In my opinion, ipfixprobe should be enabled by default or at least there should be some message that it needs to be enabled first. Documentation would be sufficient.
   Because if you don't enable it in /etc/config/iprobe and you tried to start it with /etc/init.d/ipfixprobe start it didn't do anything.

Similar to: openwrt/packages#12371

2. It would be good if there is used common local IP 192.168.1.1 for routers instead of 192.168.0.2, but this does not solve an issue if you are using it on developer/testing router where you are not using 192.168.1.1. This should be done differently most likely by looking at interface br-lan to see which IP address it is using it and assign to it by default.