The SIGNATURE_HANDLER can be defined by the user, so we can't use choices.
(Bitbake rev: 0036b36e333a68c6d30a6bf5c21bfd4fd3e8441b)
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>