After executing each ucmd, the ucmd callback function always call fastboot_setup
to setup some enviroments.

Because the mmc will be switched to user area by calling blk_get_dev in _fastboot_load_partitions.
When running "mmc partconf" by ucmd, the PART_CONFIG EXTCSD is updated, but the part_config and
hwpart variables in mmc and blk structure are not synced. So the old value will write to
PART_CONFIG EXTCSD again when switch to user area.

This patch changes the fastboot_setup, only load the partitions when the storage device is changed.
Also force to re-init mmc before loading the partitions to sync mmc variables.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit c9cd93b25586ecd4ce9178da7b8141f60cdd9deb)