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/common/Makefile b/common/Makefile
index 2e7a090..bcf352d 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -53,8 +53,7 @@
 obj-$(CONFIG_LCD_DT_SIMPLEFB) += lcd_simplefb.o
 obj-$(CONFIG_LYNXKDI) += lynxkdi.o
 obj-$(CONFIG_MENU) += menu.o
-obj-$(CONFIG_UPDATE_TFTP) += update.o
-obj-$(CONFIG_DFU_TFTP) += update.o
+obj-$(CONFIG_UPDATE_COMMON) += update.o
 obj-$(CONFIG_USB_KEYBOARD) += usb_kbd.o
 obj-$(CONFIG_CMDLINE) += cli_readline.o cli_simple.o