mmc-utils: Refactor common FFU code into functions to support additional FFU modes

Refactor common FFU code into functions to support additional FFU modes. Follow-up
patches will focus on implementing additional FFU modes and enhancements.

Signed-off-by: Bean Huo <beanhuo@micron.com>
Acked-by: Avri Altman <avri.altman@wdc.com>
Message-ID: <20241025203454.162710-2-beanhuo@iokpp.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>