UBI: move and rename attach_by_scanning

Rename the 'attach_by_scanning()' function to 'ubi_attach()' and move it to
scan.c. Richard will plug his fastmap stuff there.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

diff --cc drivers/mtd/ubi/build.c
Simple merge

diff --cc drivers/mtd/ubi/scan.c
Simple merge

diff --cc drivers/mtd/ubi/scan.h
Simple merge

diff --cc drivers/mtd/ubi/ubi.h
Simple merge