riscv: Enable SPI flash env for SiFive Unmatched.

Enable saving environment to SPI flash memory on SiFive
Unmatched.

Signed-off-by: Thomas Skibo <thomas-git@skibo.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/board/sifive/unmatched/Kconfig b/board/sifive/unmatched/Kconfig
index fb2c1fb..fe213fd 100644
--- a/board/sifive/unmatched/Kconfig
+++ b/board/sifive/unmatched/Kconfig
@@ -26,6 +26,7 @@
 config BOARD_SPECIFIC_OPTIONS # dummy
 	def_bool y
 	select SIFIVE_FU740
+	select ENV_IS_IN_SPI_FLASH
 	select SUPPORT_SPL
 	select RESET_SIFIVE
 	select BINMAN