Set eMMC boot config after flashing image
When flashing a single-file build, configure the eMMC to boot from the user area, which is where the image was just written. The eMMC boot configuration could have been changed (e.g. after flashing a multi-files image), so if it is left untouched after flashing a single-file image, the system could boot from a hwpart containing a different U-boot than the one which was just flashed. This commit prevents that by setting the eMMC to boot from the user area after flashing a single-file image there.