riscv: sifive: unleashed: Switch to use binman to generate u-boot.itb

At present SiFive Unleashed board uses the Makefile to create the FIT,
using USE_SPL_FIT_GENERATOR, which is deprecated as per the Makefile
warning. Update to use binman instead.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/board/sifive/unleashed/Kconfig b/board/sifive/unleashed/Kconfig
index dbffd59..502916e 100644
--- a/board/sifive/unleashed/Kconfig
+++ b/board/sifive/unleashed/Kconfig
@@ -27,6 +27,7 @@
 	def_bool y
 	select SIFIVE_FU540
 	select ENV_IS_IN_SPI_FLASH
+	select BINMAN
 	imply CMD_DHCP
 	imply CMD_EXT2
 	imply CMD_EXT4