cmake: rework cmocka build process

Rather than wrapping around cmocka's existing cmake, simply inherit
the build setup/variables from the waffle project and build a simple
cmocka static library.
This way we'll no longer need to add the custom commands for build
and we'll be able to easily handle mingw-w64 and msvc build :)

To make things even better this speeds up the configure/build process.

Patch is largely written by Nils Gladitz, ngladitz on #cmake
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Merge branch 'master' into next