Return None from MbedTerminal Constructor

Since we examine `self.serial` to determine the success of the
constructor, I made all `return False` statements within the constructor
set `self.serial = None`. That way the current code should catch the
failures without any additional changes

Fixes #688

Author: theotherjimmy

mbed/mbed_terminal.py

@@ -35,15 +35,16 @@ def __init__(self, port, baudrate=9600, echo=True, timeout=10):
         try:
             from serial import Serial, SerialException
         except (IOError, ImportError, OSError):
-            return False
+            self.serial = None
+            return
 
         try:
             self.serial = Serial(self.port, baudrate=self.baudrate, timeout=self.timeout)
             self.serial.flush()
             self.serial.reset_input_buffer()
         except Exception as e:
             self.serial = None
-            return False
+            return
 
     def terminal(self, print_header=True):
         try: