Makefile: Only define u-boot.itb rule when applicable

If neither CONFIG_SPL_FIT_SOURCE nor CONFIG_USE_SPL_FIT_GENERATOR is
enabled, U_BOOT_ITS will be undefined, and attempting to make u-boot.itb
will pass invalid arguments to mkimage, causing it to print its help
message.

Remove the rule in that case, so it is more obvious that u-boot.itb is
not something that can be made. This will reduce confusion as platforms
move away from CONFIG_USE_SPL_FIT_GENERATOR, as u-boot.itb was
previously a valid goal for those platforms.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>