Update to pySerial 2.7. (PR #6854, issue #6845)
This is needed to fix a UnicodeDecodeError in some languages when opening a serial port fails. This was preventing braille display drivers from trying further ports which might have worked.
pySerial is now included as its own git submodule, rather than as part of miscDeps. This necessitated adding its directory to the Python module search path in sourceEnv. Also, scons now sets the PYTHONPATH environment variable based on sourceEnv, rather than having the directories listed in two separate places.
