modularize app-OWM to not rely on Freifunk and OLSR

[SvenRoederer]

This change introduces an extension / plugin functionality, to allow running non-Freifunk systems and systems that do not use OLSR. The owm-library in split into code, that collects the basic data of a node and all other data has been moved into separate extensions. This results in the following extensions:

• interfaces: everything related to wired and wireless interfaces
• olsr: all data extreacted from olsrd (links, neighbours, config)
• freifunk: public data of the Freifunk community (uci show freifunk.public)

For the moment the extensions "freifunk" and "olsr" are non-functional / untested

[SvenRoederer]

With this extension, there is no strict dependency of the basic "luci-app-owm-cmd" package to "olsrd-mod-jsoninfo" and "luci-lib-ip" anymore. Best seems to be, to make separate packages for the extensions to allow easy selection, but this seems a bit overkill. Any suggestions for an alternative?