[MTD][MTDPART] Seperate main loop from per-partition code in add_mtd_partition

add_mtd_partition was a 150+ line monster consisting mostly of a single
loop.  Seperate the loop from most of the body.  Now it should be
obvious which variables are carried around from iteration to iteration.
Signed-off-by: Jörn Engel <joern@logfs.org>
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>