cmd: thor: select DFU subsystem also for 'thor' download tool

'THOR' download command requires DFU infrastructure to properly flash
board images. It can be used without enabling DFU command, so add such
dependency to Kconfig.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Lukasz Majewski <lukma@denx.de>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index b08a709..c414c5d 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -442,6 +442,7 @@
 
 config CMD_THOR_DOWNLOAD
 	bool "thor - TIZEN 'thor' download"
+	select DFU
 	help
 	  Implements the 'thor' download protocol. This is a way of
 	  downloading a software update over USB from an attached host.