imagetest-qemu: Allow the task to run after any rootfs is created and also standalone
One bitbake invocation can cause multiple images to be generated. We should test
each one and we cna do this by running after the rootfs task. Running the tests
standlone is also still possible with a new separate standlone task.
Acked-by: Scott Garman <scott.a.garman@intel.com> Acked-by: Jiajun Xu <jiajun.xu@intel.com> Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>
