ARM: bitmain: Enable legacy u-boot format

Still legacy formats are used that's why enable them by default.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/bitmain_antminer_s9_defconfig b/configs/bitmain_antminer_s9_defconfig
index f990448..ea21a5e 100644
--- a/configs/bitmain_antminer_s9_defconfig
+++ b/configs/bitmain_antminer_s9_defconfig
@@ -19,6 +19,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
+CONFIG_LEGACY_IMAGE_FORMAT=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_PREBOOT=y
 # CONFIG_DISPLAY_CPUINFO is not set