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: NBin Meng <bmeng.cn@gmail.com>
Signed-off-by: NJagan Teki <jagan@amarulasolutions.com>
Reviewed-by: NBin Meng <bin.meng@windriver.com>
Tested-by: NBin Meng <bin.meng@windriver.com>

board/sifive/fu540/Kconfig

@@ -27,6 +27,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	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