If user selected a base image and didn't customize it, Hob will still
build the selected image instead of hob-image.
This fixes [YOCTO #2253]
(Bitbake rev: a74a6a033e66a0ae7b47c84d842bdd54066c69a1)
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>