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/configs/dh_imx6_defconfig b/configs/dh_imx6_defconfig
index f7355d2..f0771f5 100644
--- a/configs/dh_imx6_defconfig
+++ b/configs/dh_imx6_defconfig
@@ -26,6 +26,7 @@
 # CONFIG_SYS_MALLOC_F is not set
 CONFIG_FIT=y
 CONFIG_SPL_FIT=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg"
 CONFIG_BOOTDELAY=3
 # CONFIG_USE_BOOTCOMMAND is not set