kernel-devicetree: Cope with non-standard kernel deploy subdirectory

kernel-devicetree: Cope with non-standard kernel deploy subdirectory

kernel.bbclass installs non-standard kernels (where
KERNEL_PACKAGE_NAME is not "kernel") in a subdirectory of ${DEPLOYDIR}.
To achieve this kernel_do_deploy sets the deployDir shell variable to
${DEPLOYDIR} for the standard kernel or
${DEPLOYDIR}/${KERNEL_DEPLOYSUBDIR} for non-standard kernels.

kernel-devicetree.bbclass's do_deploy_append ought to do the same
and can do so by using the same shell variable.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>