implement ubus plugin

[aparcar]

Controlling bmx7 via command line works well, but various think could be improved by using ubus instead. Various, spontaneous improvements I could think of:

• Better control of bmx7 via scripts
• Controlling bmx7 via Luci / Luci-ng / Lime-App
• Replace inotifywait with ubus wait_for (inotifywait + lib is about ~50kb in size)
• Directly announce new nodes to dnsmasq
• More!

@axn @dangowrt

[dangowrt]

• delegate mesh-acquired IPv6 prefixes to interfaces, needed to complete https://github.com/libremesh/lime-packages/pull/71

[aparcar]

@dangowrt ping