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/socfpga_arria10_defconfig b/configs/socfpga_arria10_defconfig
index 625b597..8fdd21c 100644
--- a/configs/socfpga_arria10_defconfig
+++ b/configs/socfpga_arria10_defconfig
@@ -12,6 +12,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_SPL_FIT=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200"
 # CONFIG_USE_BOOTCOMMAND is not set