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_a53_defconfig b/configs/am65x_evm_a53_defconfig
index 688fe2e..86bd940 100644
--- a/configs/am65x_evm_a53_defconfig
+++ b/configs/am65x_evm_a53_defconfig
@@ -26,6 +26,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL_LOAD_FIT=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTCOMMAND="run findfdt; run envboot; run init_${boot}; run boot_rprocs; run get_kern_${boot}; run get_fdt_${boot}; run get_overlay_${boot}; run run_kern"
 CONFIG_CONSOLE_MUX=y