this adds erase environment for mmc storage
squashed fixes:
- add CONFIG_CMD_ERASEENV
- env: erase redundant offset if defined
- changes mentioned by Simon
- fix whitespaces around errmsg
Suggested-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>