kernel.bbclass: set kerneldir

kernel.bbclass: set kerneldir

commit b6cc4bc217e695801db060a1366915a870cab5db moved work
from kernel_do_install to sysroot_stage_all_append
This change changed kerneldir and staged directy from
${SYSROOT_DESTDIR}${STAGING_KERNEL_DIR}. This works fine but
the patch removed the line kerneldir=${D}/kernel/
from do_install.

However, some kernel recipes have a do_install_append which
references ${kerneldir}.
This patch adds setting kerneldir to the end of do_install
so the value is available for everyone who appends

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>