kernel/linux-kernel-base: Fix kernel build artefact determinism issues

With the 6.1 kernel we've seen reproducibility/determinism issues where the kernel
seems to rebuild headers referencing the hostname or local user. kernel-devsrc
building after the kernel seemed to trigger it in some cases.

Moving the definitions to the bbclass used by all the kernel recipe code including
kernel-devsrc seems to be the best way to ensure this doesn't happen.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>