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/j721e_hs_evm_r5_defconfig b/configs/j721e_hs_evm_r5_defconfig
index cb6f97b..62c3bc5 100644
--- a/configs/j721e_hs_evm_r5_defconfig
+++ b/configs/j721e_hs_evm_r5_defconfig
@@ -27,6 +27,7 @@
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL_LOAD_FIT_ADDRESS=0x80080000
 CONFIG_SPL_FIT_IMAGE_POST_PROCESS=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_USE_BOOTCOMMAND=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL_STACK_R=y