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/xilinx_zynq_virt_defconfig b/configs/xilinx_zynq_virt_defconfig
index 1033ef9..8acdab2 100644
--- a/configs/xilinx_zynq_virt_defconfig
+++ b/configs/xilinx_zynq_virt_defconfig
@@ -16,6 +16,7 @@
 CONFIG_SPL_FIT_PRINT=y
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL_LOAD_FIT_ADDRESS=0x10000000
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_LEGACY_IMAGE_FORMAT=y
 CONFIG_USE_PREBOOT=y
 CONFIG_SPL_STACK_R=y