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>
diff --git a/Makefile b/Makefile
index 28c9f31..2b21675 100644
--- a/Makefile
+++ b/Makefile
@@ -1440,11 +1440,13 @@
 MKIMAGEFLAGS_u-boot.itb = -E
 endif
 
+ifdef U_BOOT_ITS
 u-boot.itb: u-boot-nodtb.bin \
 		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_OF_HOSTFILE),dts/dt.dtb) \
 		$(U_BOOT_ITS) FORCE
 	$(call if_changed,mkfitimage)
 	$(BOARD_SIZE_CHECK)
+endif
 
 u-boot-spl.kwb: u-boot.img spl/u-boot-spl.bin FORCE
 	$(call if_changed,mkimage)