license.bbclass: do write_deploy_manifest in image postprocessing

license.bbclass: do write_deploy_manifest in image postprocessing

Call write_deploy_manifest() in image postprocessing phase, instead of
rootfs postprocessing. The reason being that not necessarily all
do_deploy tasks are dependencies of the do_rootfs and we might miss
something.

[YOCTO #9446]

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>