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/mccmon6_nor_defconfig b/configs/mccmon6_nor_defconfig
index 0c24a85..7fc8a9d 100644
--- a/configs/mccmon6_nor_defconfig
+++ b/configs/mccmon6_nor_defconfig
@@ -17,6 +17,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_SPL_LOAD_FIT=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/liebherr/mccmon6/mon6_imximage_nor.cfg"
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_BOARD_LATE_INIT=y