fs: Add a driver to access U-Boot environment variables ...

Add a driver working on top of ubootvar device and exposing U-Boot
environment variable data as files.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Cory Tusar <cory.tusar@zii.aero>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>