bb/ui/hob: move some code around to avert a race

bb/ui/hob: move some code around to avert a race

The data-generated and model-updated signals are different, the model
should only be accessed *after* the model-updated signal. Move code
setting the image combo's backing model to the model-updated callback to
ensure the combo is accurately set when changing the machine with an
image selected.

(Bitbake rev: 46953d3b4595667935d43fe7b20264ae0efd1bdf)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>