PVR addons won't be loaded on pvr-nightly and imx-nightly

Because there is still no Helix branch in opdenkamps xbmc-pvr-addons, the addons will not be loaded and build on the pvr-nightly and imx-nightly platform.

A change in file hooks.d/post-clone.d/clone-pvr-addons to " Helix|master)" solves this issue