kernel-fitimage: images should not be signed with the same keys as the configurations

Otherwise the "required" property, from UBOOT_DTB_BINARY, will be set to "conf"
and no error will be raised in case of error.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>