Convert CONFIG_CMD_THOR_DOWNLOAD to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_THOR_DOWNLOAD

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/topic_miamiplus_defconfig b/configs/topic_miamiplus_defconfig
index 93e0292..18df89d 100644
--- a/configs/topic_miamiplus_defconfig
+++ b/configs/topic_miamiplus_defconfig
@@ -13,6 +13,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="zynq-uboot> "
 # CONFIG_CMD_IMLS is not set
+CONFIG_CMD_THOR_DOWNLOAD=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y