kernel-fitimage.bbclass: fix a wrong conditional check

It should check if "${UBOOT_SIGN_ENABLE}" equals to "1" instead of
checking if "${UBOOT_SIGN_ENABLE}" is not empty since it could be "0".

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 900949af)
Signed-off-by: Steve Sakoman <steve@sakoman.com>