dfu: Fix up the Kconfig mess

Clean up the screaming mess of configuration options that DFU is.
It was impossible to configure DFU such that TFTP is enabled and
USB is not, this patch fixes that and assures that DFU TFTP and
DFU USB can be enabled separatelly and that the correct pieces
of code are compiled in.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Lukasz Majewski <lukma@denx.de>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 885712f..d997f0c 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -588,7 +588,7 @@
 
 config CMD_DFU
 	bool "dfu"
-	select USB_FUNCTION_DFU
+	select DFU
 	help
 	  Enables the command "dfu" which is used to have U-Boot create a DFU
 	  class device via USB. This command requires that the "dfu_alt_info"