dfu: Migrate to Kconfig

Introduce a hidden USB_FUNCTION_DFU Kconfig option and select it for
CMD_DFU (as we must have the DFU command enabled to do anything DFU).
Make all of the entries in drivers/dfu/Kconfig depend on CMD_DFU and add
options for all of the back end choices that DFU can make use of.

Cc: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
diff --git a/configs/Sinlinx_SinA33_defconfig b/configs/Sinlinx_SinA33_defconfig
index b41bf67..2a5f985 100644
--- a/configs/Sinlinx_SinA33_defconfig
+++ b/configs/Sinlinx_SinA33_defconfig
@@ -12,6 +12,7 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_DFU=y
 # CONFIG_CMD_FPGA is not set
+CONFIG_DFU_RAM=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_MUSB_GADGET=y
 CONFIG_USB_GADGET=y