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/am65x_evm_r5_defconfig b/configs/am65x_evm_r5_defconfig
index 9e1da64..38c5557 100644
--- a/configs/am65x_evm_r5_defconfig
+++ b/configs/am65x_evm_r5_defconfig
@@ -24,6 +24,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="k3-am654-r5-base-board"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL_LOAD_FIT=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_USE_BOOTCOMMAND=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL_STACK_R=y