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/odroid-xu3_defconfig b/configs/odroid-xu3_defconfig
index ffcecd5..2354a00 100644
--- a/configs/odroid-xu3_defconfig
+++ b/configs/odroid-xu3_defconfig
@@ -12,6 +12,7 @@
 CONFIG_CONSOLE_MUX=y
 CONFIG_SYS_PROMPT="ODROID-XU3 # "
 # CONFIG_CMD_IMLS is not set
+CONFIG_CMD_THOR_DOWNLOAD=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_USB=y