Add the buffer attribute to sys.stdin, sys.stdout and sys.stderr. This
provides raw access to underlying stdio streams for the unix port (and
others that use VfsPosix).
Signed-off-by: stephanelsmith <stephane.smith@titansensor.com>
This is consistent with the other 'micro' modules and allows implementing
additional features in Python via e.g. micropython-lib's sys.
Note this is a breaking change (not backwards compatible) for ports which
do not enable weak links, as "import sys" must now be replaced with
"import usys".
