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/am57xx_evm_defconfig b/configs/am57xx_evm_defconfig
index a1153bb..7211146 100644
--- a/configs/am57xx_evm_defconfig
+++ b/configs/am57xx_evm_defconfig
@@ -18,6 +18,7 @@
 CONFIG_AHCI=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SPL_LOAD_FIT=y
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="androidboot.serialno=${serial#} console=ttyS2,115200 androidboot.console=ttyS2 androidboot.hardware=beagle_x15board"