env_mmc: add support for redundant environment

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Home-Box: enable redundant environment

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Home-Box: fix environment size/offset (do not use more space than before using redundant copies)

Signed-off-by: Michael Heimpold <mhei@heimpold.de>