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/xilinx_zynqmp_ep_defconfig b/configs/xilinx_zynqmp_ep_defconfig
index 1d5fd1f..b863c1a 100644
--- a/configs/xilinx_zynqmp_ep_defconfig
+++ b/configs/xilinx_zynqmp_ep_defconfig
@@ -21,6 +21,7 @@
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_XIMG is not set
+CONFIG_CMD_THOR_DOWNLOAD=y
 # CONFIG_CMD_EDITENV is not set
 # CONFIG_CMD_ENV_EXISTS is not set
 CONFIG_CMD_UNZIP=y