Add releng component for building with buckminster

This enables mirroring of the p2.site using p2 actions from buckminster
This file also contians extra dependencies needed to build the bundles in this repo
Removes slf4j.api from feature
Updates bundle versions to 2.6.1
Feature version set to 2.6.1
Removed version from org.slf4j

To allow for updates to 1.7.5
Merge branch '2.6.1' into develop