add 'global' command ...

This implements global shell variable support. This is done by registering
a new device named 'global', so global variables are just plain device
parameters.
Global variables are useful for storing the global state in the environment.
Currently we do this by sourcing scripts instead of executing them which is
quite limiting.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>