arm64: ls1046ardb: Add distro boot support

Tested on ls1046ardb with automatically boot Ubuntu from SD card or
USB disk, if it fails to detect external storage disk, fall back to
qspi boot.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@nxp.com>
Signed-off-by: Gong Qianyu <Qianyu.Gong@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>
diff --git a/include/configs/ls1046aqds.h b/include/configs/ls1046aqds.h
index c5c4641..891f2a3 100644
--- a/include/configs/ls1046aqds.h
+++ b/include/configs/ls1046aqds.h
@@ -461,6 +461,7 @@
 
 #define CONFIG_CMDLINE_TAG
 
+#undef CONFIG_BOOTCOMMAND
 #if defined(CONFIG_QSPI_BOOT) || defined(CONFIG_SD_BOOT_QSPI)
 #define CONFIG_BOOTCOMMAND		"sf probe && sf read $kernel_load "    \
 					"e0000 f00000 && bootm $kernel_load"