env: add ENV_ERASE_PTR macro

Add ENV_ERASE_PTR macro to handle erase opts and remove the associated
ifdef.

This patch is a extension of previous commit 82b2f4135719 ("env_internal.h:
add alternative ENV_SAVE_PTR macro").

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/include/env_internal.h b/include/env_internal.h
index 708c833..b7bddcb 100644
--- a/include/env_internal.h
+++ b/include/env_internal.h
@@ -211,6 +211,7 @@
 #endif
 
 #define ENV_SAVE_PTR(x) (CONFIG_IS_ENABLED(SAVEENV) ? (x) : NULL)
+#define ENV_ERASE_PTR(x) (CONFIG_IS_ENABLED(CMD_ERASEENV) ? (x) : NULL)
 
 extern struct hsearch_data env_htab;