env: mmc: add erase-function

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>
1 file changed