Do not run amtterm only after reset and get rid of sleep()

This way, amtterm fails is somebody is already connected. Only if amtterm
succeeds, we proceed to reset/poweron.

With recent amttool (at least commit [1]), this works even without sleep
before running amtterm.

[1]: https://www.kraxel.org/cgit/amtterm/log/?h=0ece5135fef56dbd0d94957c334655a57adb7212
AMT: Disconnect SOL before power off

Otherwise power off fails.