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>

include/environment.h

@@ -200,6 +200,7 @@ enum env_operation {
 	ENVOP_INIT,	/* we want to call the init function */
 	ENVOP_LOAD,	/* we want to call the load function */
 	ENVOP_SAVE,	/* we want to call the save function */
+	ENVOP_ERASE,	/* we want to call the erase function */
 };
 
 struct env_driver {
@@ -225,6 +226,15 @@ struct env_driver {
 	 */
 	int (*save)(void);
 
+	/**
+	 * erase() - Erase the environment on storage
+	 *
+	 * This method is optional and required for 'eraseenv' to work.
+	 *
+	 * @return 0 if OK, -ve on error
+	 */
+	int (*erase)(void);
+
 	/**
 	 * init() - Set up the initial pre-relocation environment
 	 *
@@ -303,6 +313,13 @@ int env_load(void);
  */
 int env_save(void);
 
+/**
+ * env_erase() - Erase the environment on storage
+ *
+ * @return 0 if OK, -ve on error
+ */
+int env_erase(void);
+
 /**
  * env_fix_drivers() - Updates envdriver as per relocation
  */