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.