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/am43xx_evm_usbhost_boot_defconfig b/configs/am43xx_evm_usbhost_boot_defconfig
index efe8be3..14d874f 100644
--- a/configs/am43xx_evm_usbhost_boot_defconfig
+++ b/configs/am43xx_evm_usbhost_boot_defconfig
@@ -38,6 +38,9 @@
 CONFIG_OF_CONTROL=y
 CONFIG_OF_LIST="am437x-gp-evm am437x-sk-evm am43x-epos-evm am437x-idk-evm"
 CONFIG_DM=y
+CONFIG_DFU_MMC=y
+CONFIG_DFU_RAM=y
+CONFIG_DFU_SF=y
 CONFIG_DM_GPIO=y
 CONFIG_DM_MMC=y
 CONFIG_SPI_FLASH=y