[OE-core] [PATCHv2] cmake: Avoid passing empty prefix to os.path.relpath

[Mike Crowe]

On Monday 11 December 2017 at 13:45:26 +0000, Burton, Ross wrote:
> It was implicated in some build failures.  I'll re-add it and try again.

Thanks. Are the new results in? I had a poke about on autobuilder.yocto.io
but couldn't work out how to tell which builds this commit was included in.
Build 719 did not appear to include it.

Thanks.

Mike.