net/mwan3: add hotplug script for hidden self interface generation

net/mwan3: add hotplug script for hidden self interface generation

Add new globals config section with option local_source.

With this config option the self interface generation will be done now
automatically on hotplug event. You can specify which interface (ip)
sould be used for router traffic. To replace the self intereface in the
config set local_source to "lan".

The default option is none, so it will not change default behavior if a
"self" interface is configured in the network section.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>