So we can have a persistent env and a bit safer barebox update. Also
switch the environment and update default depending on the bootsource,
as this is really helpful on an eval board where one might want to
have totally different systems on eMMC and SD card.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>