saveenv: provide a zeroed/empty/ignore environment
If an external environment storage should be used in very rare and special cases,
the intentional behaviour should be to ignore the external environment and always
fall back to the built-in environment. By storing an empty "to be ignored" environment
into the external environment a confusing error message about invalid CRC sums will go
away and still the built-in environment is used.
With this new option we can force the intentional behaviour.
Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>