kernel.bbclass: avoid NOTE messages for files that are staged but not packaged

kernel.bbclass: avoid NOTE messages for files that are staged but not packaged

copy the files directly from the work area to staging. That way
there are no unpackaged files in image/ so no NOTE messages.
Additional advantage is that it is faster as we save the copy to the image dir.

See also http://thread.gmane.org/gmane.comp.handhelds.openembedded/37487

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Acked-by: Roman I Khimov <khimov@altell.ru>
Tested-by: Petr Stetiar <ynezz@true.cz>
Acked-by: Eric Benard <eric@eukrea.com>
Acked-by: Denys Dmytriyenko <denis@denix.org>
Tested-by: Paul Menzel <paulepanter@users.sourceforge.net>