kernel.class : fix for recent kernel on x86

kernel.class : fix for recent kernel on x86

- bitbake linux for a x86 machine fails with the following message :
cp: target `.../image/kernel//include/asm-x86/' is not a directory

so create $kerneldir/include/asm-x86 before trying to copy something in
this directory.

- make headers_install fails because of missing directories in include
(tested with linux 2.6.32 and 2.6.33 on x86) and missing Kbuild

Signed-off-by: Eric Benard <eric@eukrea.com>
Acked-by: Roman I Khimov <khimov@altell.ru>