Makefile: Allow CONFIG_SPL_FIT_GENERATOR to be empty

At present we use the empty string to indicate that there is no FIT
generator, but this doesn't allow an individual board to undefine it.
Create a separate bool instead.

Update the config of the boards which currently have an empty string.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 93f2dd6..27ff8a7 100644
--- a/Makefile
+++ b/Makefile
@@ -1350,7 +1350,7 @@
 $(U_BOOT_ITS): $(subst ",,$(CONFIG_SPL_FIT_SOURCE))
 	$(call if_changed,copy)
 else
-ifneq ($(CONFIG_SPL_FIT_GENERATOR),"")
+ifneq ($(CONFIG_USE_SPL_FIT_GENERATOR),)
 U_BOOT_ITS := u-boot.its
 ifeq ($(CONFIG_SPL_FIT_GENERATOR),"arch/arm/mach-imx/mkimage_fit_atf.sh")
 U_BOOT_ITS_DEPS += u-boot-nodtb.bin