common: update: add a generic interface for FIT image

The main purpose of this patch is to separate a generic interface for
updating firmware using DFU drivers from "auto-update" via tftp.

This function will also be used in implementing UEFI capsule update
in a later commit.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/dfu/Kconfig b/drivers/dfu/Kconfig
index 10196f3..b7427fc 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -22,6 +22,7 @@
 	bool "DFU via TFTP"
 	select DFU_WRITE_ALT
 	select DFU_OVER_TFTP
+	select UPDATE_COMMON
 	help
 	  This option allows performing update of DFU-managed medium with data
 	  sent via TFTP boot.