netifd extension: external device handlers

In this project, we will extend netifd by a generic device handler interface, which is exposed via the ubus IPC system to enable interaction with external device handlers. In other words, the goal is to decouple the device management functionality from netifd to other separately maintained device managers to allow both sides to evolve independently. Moreover, the decoupling will allow netifd to manage devices of types which may even be unknown to netifd without the need to change the central UCI network device configuration in /etc/config/network. In this project, we will present a basic proof-of-concept module which outsources the control over Open vSwitch in OpenWrt to an external device handling entity. Specifically, we will realize a small daemon acting as a relay between the ovs command line tools and netifd's ubus device handler interface. Native support of Open vSwitch in OpenWRT will open a wide range of different use-cases.