sifive: fu540: Mark the default env as SPI flash

Mark the default U-Boot environment as SPI flash since
this is an on board flash device.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
diff --git a/board/sifive/fu540/Kconfig b/board/sifive/fu540/Kconfig
index 683668d..4aa1e4c 100644
--- a/board/sifive/fu540/Kconfig
+++ b/board/sifive/fu540/Kconfig
@@ -27,6 +27,7 @@
 	def_bool y
 	select SIFIVE_FU540
 	select SUPPORT_SPL
+	select ENV_IS_IN_SPI_FLASH
 	select RAM
 	select SPL_RAM if SPL
 	imply CMD_DHCP