wic: Fix kernel dir location

With the recent kernel staging changes, STAGING_KERNEL_DIR no longer
points to the kernel image, which can be found however in
DEPLOY_DIR_IMAGE.  This updates find_artifacts() to look there
instead.

Fixes [YOCTO #7307].

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>