Fixed the issue #19 and #20, adding a new function to chooseBoard

Hi, I have fixed the issue #19 and #20 I raised several days ago, sorry for the late of pull request. The main reason for me to add self.setDaemon(True) is that without this option, the gdb server thread will block your main thread in some cases, which will lead to a block in cmd line(ctrl + c won't stop the gdb_test.py)

I have also noticed that if there's more than one board connected to the PC, the chooseBoard function will be blocked until the user input the number of board they want to connect. This will be very inconvenient for auto-testing, so I added a board_id parameter to enable user to specify the board they want to connect with its unique id.