Merge branch 'linux-lts-xenial' into 'belenos'

Add helpers for linux-lts-xenial and linux-meta-lts-xenial

Mostly based off flidas `linux` and `linux-meta` helpers.

Compiled successfully for amd64 for my machine, and package installation for `-generic` works.

This package is especially relevant now, considering that the lts-wily kernel is no longer supported.

See merge request !159