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/am335x_boneblack_defconfig b/configs/am335x_boneblack_defconfig
index a3994f7..2ef1aff 100644
--- a/configs/am335x_boneblack_defconfig
+++ b/configs/am335x_boneblack_defconfig
@@ -34,6 +34,8 @@
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_DFU_TFTP=y
+CONFIG_DFU_MMC=y
+CONFIG_DFU_RAM=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_SYS_NS16550=y