Add a driver to expose U-Boot environment variable data as a single
mmap-able device, hiding various low-level details such as:

    * Preamble format differences
    * Read/write logic in presence of redundant partition

Not very useful on its own, it is a crucial low-level plumbing needed
by filesystem driver introduced in the following commit.

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>