Mock Turtle Version 3.1.0

** [3.1.0] - 2018-03-09
This release brings in the gateware from wr-node-core repository, such as it
is being used in the released and deployed v1.0 version of masterFIP.
The relevant commit is 96a7859 from git://ohwr.org/white-rabbit/wr-node-core.git.
This is a "known to work" version of the gateware, still using the "node core" name.
Following releases will switch to the "mock turtle" name.
This tag has no release notes.