[backend] publish ONIE binary

Open Network Install Environment is an open image format used by
networking vendor to ship a standardised image for networking white
box switches.

Those images names have a standard suffix, "ONIE.bin", which means
right now they are ignored by the OBS publisher when they are built.
Add a simple match for this new type, and publish into an onie/
subdirectory.

Reference: http://onie.org