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>