kernel-dev: Added KMACHINE and MACHINE descriptions for using metadata

kernel-dev: Added KMACHINE and MACHINE descriptions for using metadata

Needed to remove an old example that was showing how two BSPs could
share the same KMACHINE value.  I replaced it with corei7-based
machines.  Also noted that KMACHINE is for kernel mapping only
whereas MACHINE is for the layer's BSP machine type.

(From yocto-docs rev: c1dad2815b6fd1ed78e0dce0fecb422f342eaf14)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>