uboot-sign: Fix using wrong KEY_REQ_ARGS

When generating our SPL-verifying certificate, we use FIT_KEY_REQ_ARGS,
which is intended for the U-Boot-verifying certificate. Instead, use
UBOOT_FIT_KEY_REQ_ARGS.

Fixes: 0e6b0fef ("u-boot: Use a different Key for SPL signing")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>