Allow to cancel/stop building and flashing

Sometimes you want I choose wrong settings or I see that flashing won't work because IP is wrong and I want to cancel it and try again but there's no button. Currently you need to restart configurator or wait until build and multiple flash retries are done.

Here's two images of when I might want to cancel. Of course you shouldn't be able to cancel flashing in the middle of it because it will corrupt firmware, but if it's not even starting then I would like to cancel retries