nebula: implement netifd support

This commit contains the following:
* Update binary to version 1.6.1
* Update README URLs in the Makefile to link OpenWrt-specific info
* Separate the binary, the init script and netifd script into 3 packages:
  nebula, nebula-service and nebula-proto accordingly
* implement yml parser for init script to fetch variables from it
* add the netifd script for nebula protocol
* update test file to address all built packages
* make the PKG_VERSION variable of init/proto scripts readonly

Signed-off-by: Stan Grishin <stangri@melmac.ca>