make env_entry::callback conditional on !CONFIG_SPL_BUILD

The callback member of struct env_entry is always NULL for an SPL
build. Removing it thus saves a bit of run-time memory in the
SPL (when CONFIG_SPL_ENV_SUPPORT=y) since struct env_entry is embedded
in struct env_entry_node - i.e. about 2KB for the normal case of
512+change hash table entries.

Two small fixups are needed for this, all other references to the
callback member are already under !CONFIG_SPL_BUILD: Don't initialize
.callback in set_flags() - hsearch_r doesn't use that value
anyway. And make env_callback_init() initialize ->callback to NULL for
a new entry instead of relying on an unused or deleted entry having
NULL in ->callback.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>

lib/hashtable.c

@@ -450,7 +450,6 @@ static void _hdelete(const char *key, struct hsearch_data *htab,
 	debug("hdelete: DELETING key \"%s\"\n", key);
 	free((void *)ep->key);
 	free(ep->data);
-	ep->callback = NULL;
 	ep->flags = 0;
 	htab->table[idx].used = USED_DELETED;