env: register erase command

env: register erase command

this patch adds basic changes for adding a erase-subcommand to env

with this command the environment stored on non-volatile storage written
by saveenv can be cleared.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
squashed fixes
 - start message with "Erasing"
 - mark erase-function as optional
 - env: separate eraseenv from saveenv

Suggested-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>