Add a way to have a wait for start or prompt

[MrKevinWeiss]

Since there may be times where a command is not issued but a response is needed a call should be added to receive something without sending a command. This may also have a longer timeout. The use case is when flashing a reset is needed, some boards take up to 5 seconds to be able to respond. Instead of sleep(5) with would be nice to have wait_for_startup(5) that could stop sooner than 5 seconds if the board is ready.