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>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 67284d8..ec746fe 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -408,6 +408,14 @@
 	  Save all environment variables into the compiled-in persistent
 	  storage.
 
+config CMD_ERASEENV
+	bool "eraseenv"
+	default n
+	depends on CMD_SAVEENV
+	help
+	  Erase environment variables from the compiled-in persistent
+	  storage.
+
 config CMD_ENV_EXISTS
 	bool "env exists"
 	default y