lib/oe/classextend.py: avoid extending any kernel package
For multilib and other uses of classextend, we don't want any
dependencies on kernel packages to be extended since there should only
be one kernel variant.
Fixes [YOCTO #2918] (where kernel-dev was being extended.)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>