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/Kconfig b/Kconfig
index c2cba69..92a8a69 100644
--- a/Kconfig
+++ b/Kconfig
@@ -604,9 +604,13 @@
 	  U-Boot FIT image. This could specify further image to load and/or
 	  execute.
 
+config USE_SPL_FIT_GENERATOR
+	bool "Use a script to generate the .its script"
+	default y if SPL_FIT
+
 config SPL_FIT_GENERATOR
 	string ".its file generator script for U-Boot FIT image"
-	depends on SPL_FIT
+	depends on USE_SPL_FIT_GENERATOR
 	default "board/sunxi/mksunxi_fit_atf.sh" if SPL_LOAD_FIT && ARCH_SUNXI
 	default "arch/arm/mach-rockchip/make_fit_atf.py" if SPL_LOAD_FIT && ARCH_ROCKCHIP
 	default "arch/arm/mach-zynqmp/mkimage_fit_atf.sh" if SPL_LOAD_FIT && ARCH_ZYNQMP