ARM: bitmain: Enable saving variables to SD card

Board has NAND and SD interfaces which can be used for saving variables
too.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/bitmain_antminer_s9_defconfig b/configs/bitmain_antminer_s9_defconfig
index ea21a5e..76cccfa 100644
--- a/configs/bitmain_antminer_s9_defconfig
+++ b/configs/bitmain_antminer_s9_defconfig
@@ -51,6 +51,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_ENV_OVERWRITE=y
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_NET_RANDOM_ETHADDR=y