lib/string_choices: Add str_true_false()/str_false_true() helper

Add str_true_false()/str_false_true() helper to return "true" or
"false" string literal.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20240827024517.914100-2-lihongbo22@huawei.com


Signed-off-by: Kees Cook <kees@kernel.org>