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: NMichal Simek <michal.simek@xilinx.com>

configs/bitmain_antminer_s9_defconfig

@@ -51,6 +51,7 @@ CONFIG_CMD_EXT4_WRITE=y
 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