ci/poe-powered: allow setting a different boot timeout than 5min, and shorten it to 90sec on the rpi farm
5 minutes is too long and doesn't leave enough time after the retry to still complete the job, especially if two boots fail in a row, removing 10 out of the worst-case 20 minutes allowed.
90 seconds is more than enough, the boot output normally starts after ~40-45 seconds.
Examples of this specific failure: