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/display5_factory_defconfig b/configs/display5_factory_defconfig
index e0b48a4..622cf52 100644
--- a/configs/display5_factory_defconfig
+++ b/configs/display5_factory_defconfig
@@ -26,6 +26,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="imx6q-display5"
 CONFIG_FIT=y
 CONFIG_SPL_LOAD_FIT=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg,MX6Q"
 CONFIG_BOOTDELAY=3