[oe] Cmake cross-compile file

[Matthew Dombroski]

>
> I had already posted this patch earlier bit I got no comment. It allows cmake
> to work with cross-compile files where the needed paths, options and programs
> are given, which is the recommended way of doing cross-compilation with cmake
> according to their documentation.
>
> Furthermore, you can add additionnal path in individual recipes by writing into
> the cross-compile file in the do_generate_toolchain_file_append() task as done
> below for qt4 for instance.
>
>
>
> I have been using it in my overlay for quite some time with various cmake based
> recipes we have here and I definitely think it would deserve OE inlcusion.
>
>    

I have been using the same system for cmake too, with one small change - 
I have made the toolchain path configurable.
OECMAKE_TOOLCHAIN_FILE ?= "${WORKDIR}/toolchain.cmake"

~Matthew