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/zynq_zybo_defconfig b/configs/zynq_zybo_defconfig
index e5ecb79..f2c0f72 100644
--- a/configs/zynq_zybo_defconfig
+++ b/configs/zynq_zybo_defconfig
@@ -30,6 +30,8 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_DM_SEQ_ALIAS=y
+CONFIG_DFU_MMC=y
+CONFIG_DFU_RAM=y
 CONFIG_ZYNQ_SDHCI=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_BAR=y