Pyocd gdb server improvements

Hi, as I mentioned in my former pull request, I build a enhanced gdb server based on gdb_test. I'm not sure if pyocd can include this test case. The gdb_server.py provide the following options, which can largely facilitate user experience:

"-p", "--port", help = "Write the port number that GDB server will open"
"-b", "--board", help = "Write the board id you want to connect"
"-l", "--list", help = "List all the connected board"
"-d", "--debug", help = "Set the level of system logging output, the available value for DEBUG_LEVEL: debug, info, warning, error, critical" )

So this pull request contains a new function listConnectedBoards for "-l" option, and the "-b" option's interface has already contained in my former pull request(board_id parameter in chooseBoard).

Finally, the modification to the:
```
print "%d => %s" % (index, mbed.getInfo().encode('ascii', 'ignore'))
````
is because that print the mbed.getInfo() directly will leads to some syntax error on some version of Linux.

BR,
Tony