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/zynq_zybo_defconfig b/configs/zynq_zybo_defconfig
index 4392361..31499a1 100644
--- a/configs/zynq_zybo_defconfig
+++ b/configs/zynq_zybo_defconfig
@@ -14,6 +14,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Zynq> "
 # CONFIG_CMD_IMLS is not set
+CONFIG_CMD_THOR_DOWNLOAD=y
 CONFIG_CMD_EEPROM=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y