[CMake] linux kernel module, output directory and clean issues

[Jack Kelly]

Tim Schooley wrote:
> My second problem: I'm aware that you currently cant add custom commands
> to the "clean" target, but is there _any_ way of cleaning up the kernel
> module source directory when "clean" is called, even if it's a slightly
> nasty hack ?

Do an ADD_CUSTOM_TARGET for something like `mrproper'? Not quite clean, 
but close. Besides, the linux kernel makefiles have a mrproper target.

-- Jack