dfu: Make DFU_TFTP depend on NETDEVICES

In order to do a DFU update over TFTP we need to have some network
device available, so make this depend on NETDEVICES

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/dfu/Kconfig b/drivers/dfu/Kconfig
index 4e80e85..8771678 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -19,6 +19,7 @@
 
 config DFU_TFTP
 	bool "DFU via TFTP"
+	depends on NETDEVICES
 	select UPDATE_COMMON
 	select DFU_OVER_TFTP
 	help