uboot-imx

test/py: only check for SPL signature if SPL uses serial output

check for U-Boot SPL signature only if SPL really has a serial output.
So check if CONFIG_SPL_SERIAL_SUPPORT is active in board config.

Signed-off-by: Heiko Schocher <hs@denx.de>
Tested-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>