[OE-core] GCC 8 hosts

[Burton, Ross]

Hi,

Just thought I'd say that GCC 8 is going to start appearing in the
wild, I believe the first distro to ship with it will be Fedora 28.
As usual there are changes in behaviour so there are several problems.
So far I've identified the following issues:

* Python 2 crashes on startup
* Kernel tools fail to build
* grub fails to build
* ovmf fails to build

I've patches locally for all but the ovmf failure (which is a pain
because the source uses CRLF) so if anyone switches to a GCC 8 host
before they're merged and wants to help, you know where to find me.

Ross