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/am43xx_evm_defconfig b/configs/am43xx_evm_defconfig
index e3c46a8..0bac68e 100644
--- a/configs/am43xx_evm_defconfig
+++ b/configs/am43xx_evm_defconfig
@@ -11,6 +11,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="am437x-gp-evm"
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SPL_LOAD_FIT=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_MISC_INIT_R is not set