hob: changes to image combo box from image configuration page

In some cases when a new machine is selected, the image combo
shows the same option. Fixed this issue, because the image
combo box should be reseted.

Fix the counter for the options in the combobox. This bug
was introduced by the templates functionality. The combo box had some
last changes, and I forgot about this counter.

[YOCTO #4858 & #5000]
Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>