env: sf: add support for env erase

One new function env_sf_erase() is added to support clearing flash
region(s) holding environment configuration.

It treats redundant configuration properly when U-Boot config option
SYS_REDUNDAND_ENVIRONMENT is active.

Unfortunately, none of the #if CONFIG_IS_ENABLED() sections can be
avoided.

Signed-off-by: Harry Waschkeit <Harry.Waschkeit@men.de>