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_hs_evm_a53_defconfig b/configs/am65x_hs_evm_a53_defconfig
index 04734ef..459fa6d 100644
--- a/configs/am65x_hs_evm_a53_defconfig
+++ b/configs/am65x_hs_evm_a53_defconfig
@@ -29,6 +29,7 @@
 CONFIG_FIT_IMAGE_POST_PROCESS=y
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL_FIT_IMAGE_POST_PROCESS=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTCOMMAND="run findfdt; run envboot; run init_${boot}; run get_fit_${boot}; run get_overlaystring; run run_fit"
 CONFIG_CONSOLE_MUX=y