kernel.bbclass: include signing keys when copying files required for module builds

kernel.bbclass: include signing keys when copying files required for module builds

The absence of signing_key.* in $kerneldir made signing of
out-of-tree kernel modules fail (silently). Add copying of these
files during the shared_workdir task.

Signed-off-by: Mattias Waldo <mattias.waldo@saabgroup.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>