After a lot of searching – and more testing (mostly unsuccessful..) I finally have a set of two patches that fix the Ethernet driver problem with the Rock2 in 4.6-rc1.. The problem is that the stmmac driver is common to several SOC setups, and each maintainer seems to have produced their own version of a fix..
The first patch is described here : http://lkml.iu.edu/hypermail/linux/kernel/1603.3/00194.html
The second is here: http://www.spinics.net/lists/netdev/msg369463.html
After applying both of these to 4.6-rc1, eth0 is now present, and network traffic works..
Robert Gadsdon. March 31, 2016.