TF700T: Add another unmount command prior to format /system partition

When downgrade from jb to cm10.1 flashing the ota file, some of the files
in /system are not written. As result of this, system is inconsistent when
system starts. For example, I notice that for example libjni_latinime.so is
the same of cm10.1 after the flashing, which causes continous FCs in AOSP
keyboard.
Adding an extra unmount command to updater-script prior to format /system
partition do the trick of fixing the related problem.

run_program("/tmp/backuptool.sh", "backup");
unmount("/system");
show_progress(0.500000, 0);
unmount("/system");  <<---
format("ext4", "EMMC", "/dev/block/mmcblk0p1", "0", "/system");
mount("ext4", "EMMC", "/dev/block/mmcblk0p1", "/system");

Change-Id: I80936ebbc24e2648b9408a2ca052233872e32487
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>