[oe] iproute2 - meta-openembedded/libmnl and Yocto Project compat

[Mark Hatle]

The patch to oe-core:

commit 944ef0de241de77429ab0e5cb1dd4a7f355cf3fd
Author: Yi Zhao <yi.zhao at windriver.com>
Date:   Tue Aug 15 15:38:36 2017 +0800

    iproute2: add tipc support

added tipc support.  This is part of the work that originated in
meta-openembedded.  However, the default of enabling tipc was not done as it
requires libmnl, which is only available meta-networking... and we didn't catch
this until after feature freeze.

So in meta-openembedded -- there is a single .bbappend to iproute2 that sets
PACKAGECONFIG_append = " tipc"

(Without this change, the tipc-utils fails to build, because it requires
iproute2-tipc.  Unfortunately there is no way for tipc-utils to check the
configuration of iproute2 to make this dependency required or not.)

This works -- but this type of thing is forbidden in the yocto-compat-layer.py
script.

The only way I can see to resolve this issue is to move libmnl into oe-core, as
well as make the tipc setting the default for iproute2.

I'm open to alternatives, but I'm kind of stuck here otherwise.

--Mark