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>