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 82b2f41357 ("env_internal.h:
add alternative ENV_SAVE_PTR macro").

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>

env/ext4.c

@@ -188,6 +188,5 @@ U_BOOT_ENV_LOCATION(ext4) = {
 	ENV_NAME("EXT4")
 	.load		= env_ext4_load,
 	.save		= ENV_SAVE_PTR(env_ext4_save),
-	.erase		= CONFIG_IS_ENABLED(CMD_ERASEENV) ? env_ext4_erase :
-							    NULL,
+	.erase		= ENV_ERASE_PTR(env_ext4_erase),
 };